Display device

ABSTRACT

A display device with excellent display quality is provided. The display device includes a transistor over a first substrate, an inorganic insulating film in contact with the transistor, and an organic insulating film in contact with the inorganic insulating film. The transistor includes a gate electrode over the first substrate, an oxide semiconductor film overlapping with the gate electrode, a gate insulating film in contact with one surface of the oxide semiconductor film, and a pair of electrodes in contact with the oxide semiconductor film. The inorganic insulating film is in contact with the other surface of the oxide semiconductor film. The organic insulating film overlaps with the oxide semiconductor film with the inorganic insulating film provided therebetween and is separated. Note that the thickness of the organic insulating film is preferably greater than or equal to 500 nm and less than or equal to 10 μm.

TECHNICAL FIELD

The present invention relates to an object, a method, or a manufacturingmethod. In addition, the present invention relates to a process, amachine, manufacture, or a composition of matter. One embodiment of thepresent invention particularly relates to a semiconductor device, adisplay device, a light-emitting device, a power storage device, adriving method thereof, or a manufacturing method thereof. Specifically,one embodiment of the present invention relates to a display device anda manufacturing method thereof

BACKGROUND ART

Attention has been focused on a technique for forming a transistor usinga semiconductor thin film formed over a substrate (also referred to as athin film transistor (TFT)). Such a transistor is applied to a widerange of electronic devices such as an integrated circuit (IC) or animage display device (display device). A silicon-based semiconductormaterial is widely known as a material for a semiconductor thin filmapplicable to a transistor. As another material, an oxide semiconductorhas been attracting attention.

For example, a transistor including an oxide semiconductor containingindium (In), gallium (Ga), and zinc (Zn) as an active layer is disclosed(see Patent Document 1).

In addition, a technique for improving carrier mobility by forming astack of oxide semiconductor films that is used for an active layer of atransistor has been disclosed (see Patent Document 2).

It has been pointed out that by entry of impurities such as hydrogen, anelectrically shallow donor level is formed and electrons to be carriersare generated in an oxide semiconductor. As a result, the thresholdvoltage of a transistor including an oxide semiconductor is shifted inthe negative direction and the transistor becomes normally-on, so thatleakage current in a state where voltage is not applied to the gate(that is, in the off state) is increased. Thus, the entry of hydrogeninto an oxide semiconductor film is suppressed by providing an aluminumoxide film having a property of blocking hydrogen over the entire regionof a substrate so as to cover a channel region in the oxidesemiconductor film, a source electrode, and a drain electrode, so thatgeneration of leakage current is suppressed (see Patent Document 3).

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2006-165528-   [Patent Document 2] Japanese Published Patent Application No.    2011-138934-   [Patent Document 3] Japanese Published Patent Application No.    2010-016163

DISCLOSURE OF INVENTION

A transistor including an oxide semiconductor film has a problem in thatthe amount of change in electrical characteristics, typically thresholdvoltage, due to change over time or a stress test is increased. Atransistor having normally-on characteristic causes various problems inthat power consumption is increased when the transistor is not inoperation or that display quality is reduced due to a decrease incontrast of a display device, for example.

Thus, an object of one embodiment of the present invention is to providea display device with excellent display quality. Another object of oneembodiment of the present invention is to provide a display devicehaving a high aperture ratio and including a capacitor which canincrease charge capacity. Another object of one embodiment of thepresent invention is to provide a display device with low powerconsumption. Another object of one embodiment of the present inventionis to provide a display device including a transistor having excellentelectrical characteristics. Another object of one embodiment of thepresent invention is to provide a novel display device. Another objectof one embodiment of the present invention is to provide a method formanufacturing a display device having a high aperture ratio and a wideviewing angle in fewer steps. Another object of one embodiment of thepresent invention is to provide a novel method for manufacturing adisplay device.

Note that the descriptions of these objects do not disturb the existenceof other objects. In one embodiment of the present invention, there isno need to achieve all the objects. Other objects will be apparent fromand can be derived from the description of the specification, thedrawings, the claims, and the like.

According to one embodiment of the present invention, a display deviceincludes a transistor over a first substrate, an inorganic insulatingfilm in contact with the transistor, and an organic insulating film incontact with the inorganic insulating film. The transistor includes agate electrode over the first substrate, an oxide semiconductor filmoverlapping with the gate electrode, a gate insulating film in contactwith one surface of the oxide semiconductor film, and a pair ofelectrodes in contact with the oxide semiconductor film. The inorganicinsulating film is in contact with the other surface of the oxidesemiconductor film. Note that the other surface of the oxidesemiconductor film may be an upper surface of the oxide semiconductorfilm. The organic insulating film overlaps with the oxide semiconductorfilm with the inorganic insulating film provided therebetween and isisolated. Note that the thickness of the organic insulating film ispreferably greater than or equal to 500 nm and less than or equal to 10μm. Moreover, an end portion of the gate electrode is preferablypositioned on an outer side of an end portion of the organic insulatingfilm. Alternatively, an end portion of the gate electrode does notpreferably overlap with the organic insulating film. Moreover, in a planview, the organic insulating film may completely overlap with the oxidesemiconductor film.

Note that the display device may further include a second substrateoverlapping with the first substrate, the transistor and the organicinsulating film between the first substrate and the second substrate,and a liquid crystal layer between the organic insulating film and thesecond substrate.

Alternatively, the display device may further include the secondsubstrate overlapping with the first substrate, the transistor and theorganic insulating film between the first substrate and the secondsubstrate, and liquid crystal layer is not provided between the organicinsulating film and the second substrate. In this case, the organicinsulating film functions as a spacer for holding space between thefirst substrate and the second substrate.

The inorganic insulating film may include an oxide insulating film incontact with the other surface of the oxide semiconductor film and anitride insulating film in contact with the oxide insulating film.

The display device may further include a pixel electrode connected toone of a pair of electrodes. In this case, the pixel electrode is formedusing a light-transmitting conductive film. The display device mayfurther include a metal oxide film formed in contact with the gateinsulating film and the inorganic insulating film and overlapping withthe pixel electrode with the inorganic insulating film providedtherebetween. Note that an upper surface of the metal oxide film may bein contact with the inorganic insulating film. Note that the metal oxidefilm contains the same metal element as the oxide semiconductor film.Furthermore, the pixel electrode, the inorganic insulating film, and themetal oxide film function as a capacitor.

Alternatively, the pixel electrode may be a metal oxide film formed overthe gate insulating film and containing the same metal element as theoxide semiconductor film. In this case, the display device furtherincludes a light-transmitting conductive film overlapping with the pixelelectrode with the inorganic insulating film provided therebetween, andthe light-transmitting conductive film functions as a common electrode.Furthermore, the pixel electrode, the inorganic insulating film, and thelight-transmitting conductive film function as a capacitor.

The oxide semiconductor film may contain an In—Ga oxide, In—Zn oxide, oran In-M-Zn oxide (M is Al, Ga, Y, Zr, Sn, La, Ce, or Nd). Alternatively,the oxide semiconductor film may have a multilayer structure of a firstfilm and a second film, and the first film may differ from the secondfilm in atomic ratio of metal elements.

According to one embodiment of the present invention, a display devicewith excellent display quality can be provided. A display device havinga high aperture ratio and including a capacitor which can increasecharge capacity can be provided. A display device with low powerconsumption can be provided. A display device including a transistorhaving excellent electrical characteristics can be provided. A displaydevice having a high aperture ratio and a wide viewing angle in fewersteps can be manufactured. A novel display device can be provided. Notethat the description of these effects does not disturb the existence ofother effects. One embodiment of the present invention does notnecessarily achieve all the objects listed above. Other effects will beapparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A to 1E are a top view and cross-sectional views illustrating oneembodiment of a semiconductor device.

FIGS. 2A and 2B are cross-sectional views illustrating one embodiment ofa semiconductor device.

FIGS. 3A to 3C are a block diagram and circuit diagrams illustrating oneembodiment of a display device.

FIG. 4 is a top view illustrating one embodiment of a display device.

FIG. 5 is a cross-sectional view illustrating one embodiment of adisplay device.

FIG. 6 is a cross-sectional view illustrating one embodiment of adisplay device.

FIG. 7 is a cross-sectional view illustrating one embodiment of adisplay device.

FIG. 8 is a cross-sectional view illustrating one embodiment of adisplay device.

FIG. 9 is a cross-sectional view illustrating one embodiment of adisplay device.

FIGS. 10A to 10D are cross-sectional views illustrating one embodimentof a method for manufacturing a display device.

FIGS. 11A to 11D are cross-sectional views illustrating one embodimentof a method for manufacturing a display device.

FIGS. 12A to 12C are cross-sectional views illustrating one embodimentof a method for manufacturing a display device.

FIG. 13 is a cross-sectional view illustrating one embodiment of amethod for manufacturing a display device.

FIGS. 14A and 14B are a top view and a cross-sectional view illustratingone embodiment of a display device.

FIG. 15 is a top view illustrating one embodiment of a display device.

FIG. 16 is a top view illustrating one embodiment of a display device.

FIG. 17 is a top view illustrating one embodiment of a display device.

FIG. 18 is a top view illustrating one embodiment of a display device.

FIG. 19 is a top view illustrating one embodiment of a display device.

FIG. 20 is a cross-sectional view illustrating one embodiment of adisplay device.

FIGS. 21A to 21C are cross-sectional views illustrating one embodimentof a method for manufacturing a display device.

FIGS. 22A to 22C are cross-sectional views illustrating one embodimentof a method for manufacturing a display device.

FIG. 23 is a cross-sectional view illustrating one embodiment of adisplay device.

FIG. 24 is a cross-sectional view illustrating one embodiment of adisplay device.

FIGS. 25A to 25C are cross-sectional views illustrating one embodimentof a method for manufacturing a display device.

FIGS. 26A and 26B are each a cross-sectional view illustrating oneembodiment of a display device.

FIGS. 27A to 27C are cross-sectional TEM images and a local Fouriertransform image of an oxide semiconductor.

FIGS. 28A and 28B show nanobeam electron diffraction patterns of oxidesemiconductor films and FIGS. 28C and 28D illustrate an example of atransmission electron diffraction measurement apparatus.

FIG. 29A shows an example of structural analysis by transmissionelectron diffraction measurement and FIGS. 29B and 29C show plan-viewTEM images.

FIGS. 30A and 30B are conceptual diagrams illustrating examples of adriving method of a display device.

FIG. 31 illustrates a display module.

FIGS. 32A to 32D are external views each illustrating one mode of anelectronic device.

FIG. 33 is a cross-sectional view illustrating one embodiment of adisplay device.

FIG. 34 is a graph showing temperature dependence of conductivity.

FIG. 35 shows a change in crystal parts by electron beam irradiation.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described below in detailwith reference to the drawings. Note that the present invention is notlimited to the following description, and it is easily understood bythose skilled in the art that the mode and details can be variouslychanged without departing from the spirit and scope of the presentinvention. Therefore, the present invention should not be construed asbeing limited to the description in the following embodiments. Inaddition, in the following embodiments, the same portions or portionshaving similar functions are denoted by the same reference numerals orthe same hatching patterns in different drawings, and descriptionthereof will not be repeated.

Note that in each drawing described in this specification, the size, thefilm thickness, or the region of each component is exaggerated forclarity in some cases. Therefore, embodiments of the present inventionare not limited to such a scale.

In addition, terms such as “first”, “second”, and “third” in thisspecification are used in order to avoid confusion among components, andthe terms do not limit the components numerically. Therefore, forexample, the term “first” can be replaced with the term “second”,“third”, or the like as appropriate.

Functions of a “source” and a “drain” are sometimes replaced with eachother when the direction of current flow is changed in circuitoperation, for example. Therefore, the terms “source” and “drain” can beused to denote the drain and the source, respectively, in thisspecification.

Note that a voltage refers to a difference between potentials of twopoints, and a potential refers to electrostatic energy (electricpotential energy) of a unit charge at a given point in an electrostaticfield. Note that in general, a difference between a potential of onepoint and a reference potential is merely called a potential or avoltage, and a potential and a voltage are used as synonymous words inmany cases. Thus, in this specification, a potential may be rephrased asa voltage and a voltage may be rephrased as a potential unless otherwisespecified.

In this specification, a term “parallel” indicates that the angle formedbetween two straight lines is greater than or equal to −10° and lessthan or equal to 10°, and accordingly also includes the case where theangle is greater than or equal to −5° and less than or equal to 5°. Inaddition, a term “perpendicular” indicates that the angle formed betweentwo straight lines is greater than or equal to 80° and less than orequal to 100°, and accordingly includes the case where the angle isgreater than or equal to 85° and less than or equal to 95°.

In this specification, trigonal and rhombohedral crystal systems areincluded in a hexagonal crystal system.

Embodiment 1

In this embodiment, a semiconductor device which is one embodiment ofthe present invention is described with reference to drawings.

FIGS. 1A to 1C are a top view and cross-sectional views of a transistor10 included in a semiconductor device. FIG. 1A is a top view of thetransistor 10, FIG. 1B is a cross-sectional view taken alongdashed-dotted line A-B in FIG. 1A, and FIG. 1C is a cross-sectional viewtaken along dashed-dotted line C-D in FIG. 1A. Note that in FIG. 1A, afirst substrate 11, a gate insulating film 14, an inorganic insulatingfilm 30, and the like are omitted for simplicity.

The transistor 10 illustrated in FIGS. 1A to 1C is a channel-etchedtransistor and includes a conductive film 13 functioning as a gateelectrode provided over the first substrate 11, the gate insulating film14 formed over the first substrate 11 and the conductive film 13functioning as a gate electrode, an oxide semiconductor film 19 aoverlapping with the conductive film 13 functioning as a gate electrodewith the gate insulating film 14 provided therebetween, and conductivefilms 21 a and 21 b functioning as a source electrode and a drainelectrode in contact with the oxide semiconductor film 19 a. A firstinsulating film is provided over the gate insulating film 14, the oxidesemiconductor film 19 a, and the conductive films 21 a and 21 b, and asecond insulating film overlapping with the oxide semiconductor film 19a is provided over the first insulating film.

The total thickness of the first insulating film and the secondinsulating film is preferably a thickness at which electric charges arenot generated on the surface of the second insulating film when avoltage is applied to the conductive film 13 functioning as a gateelectrode, typically a thickness greater than or equal to 600 nm. Notethat in order to reduce the amount of defects at an interface with theoxide semiconductor film 19 a, the first insulating film is preferablyan inorganic insulating film, typically an oxide insulating film isincluded. In order to shorten the process time, the second insulatingfilm is preferably an organic insulating film. In the followingdescription, the inorganic insulating film 30 is used as the firstinsulating film, and an organic insulating film 31 is used as the secondinsulating film. Moreover, an end portion of the gate electrode ispositioned on an outer side of an end portion of the organic insulatingfilm. Alternatively, an end portion of the gate electrode does notoverlap with the organic insulating film.

The inorganic insulating film 30 includes at least an oxide insulatingfilm, and the oxide insulating film is preferably stacked with a nitrideinsulating film. The oxide insulating film is formed in a region of theinorganic insulating film 30 which is in contact with the oxidesemiconductor film 19 a, whereby the amount of defects at an interfacebetween the oxide semiconductor film 19 a and the inorganic insulatingfilm 30 can be reduced.

The nitride insulating film functions as a barrier film against water,hydrogen, or the like. When water, hydrogen, or the like enters theoxide semiconductor film 19 a, oxygen contained in the oxidesemiconductor film 19 a reacts with water, hydrogen, or the like andtherefore an oxygen vacancy is formed in the oxide semiconductor film 19a. Furthermore, when carriers are generated in the oxide semiconductorfilm 19 a by oxygen vacancies, the threshold voltage of the transistorshifts in the negative direction; accordingly, the transistor hasnormally-on characteristic. Therefore, by providing a nitride insulatingfilm as a portion of the inorganic insulating film 30, the diffusionamount of water, hydrogen, or the like from the outside to the oxidesemiconductor film 19 a can be reduced and thus the amount of defects inthe oxide semiconductor film 19 a can be reduced. Accordingly, an oxideinsulating film and a nitride insulating film are stacked in this orderon the oxide semiconductor film 19 a side in the inorganic insulatingfilm 30, whereby the amount of defects at the interface between theoxide semiconductor film 19 a and the inorganic insulating film 30 andthe amount of oxygen vacancies in the oxide semiconductor film 19 a canbe reduced and therefore a transistor having normally-off characteristiccan be manufactured.

Furthermore, in the transistor 10 shown in this embodiment, the isolatedorganic insulating film 31 overlaps with the oxide semiconductor film 19a over the inorganic insulating film 30.

The thickness of the organic insulating film 31 is preferably greaterthan or equal to 500 nm and less than or equal to 10 μm.

The organic insulating film 31 is formed using an organic resin such asan acrylic resin, a polyimide resin, or an epoxy resin.

Here, the case where a negative voltage is applied to the conductivefilm 13 functioning as a gate electrode when the organic insulating film31 is not formed over the inorganic insulating film 30 is described withreference to FIG. 2B.

When a negative voltage is applied to the conductive film 13 functioningas a gate electrode, an electric field is generated. The electric fieldis not blocked with the oxide semiconductor film 19 a and affects theinorganic insulating film 30; therefore, the surface of the inorganicinsulating film 30 is weakly positively charged. Moreover, when anegative voltage is applied to the conductive film 13 functioning as agate electrode, positively charged particles contained in the air areadsorbed on the surface of the inorganic insulating film 30 and weakpositive electric charges are generated on the surface of the inorganicinsulating film 30.

The surface of the inorganic insulating film 30 is positively charged,so that an electric field is generated and the electric field affectsthe interface between the oxide semiconductor film 19 a and theinorganic insulating film 30. Thus, the interface between the oxidesemiconductor film 19 a and the inorganic insulating film 30 issubstantially in a state to which a positive bias is applied andtherefore the threshold voltage of the transistor shifts in the negativedirection.

On the other hand, the transistor 10 illustrated in FIG. 2A in thisembodiment includes the organic insulating film 31 over the inorganicinsulating film 30. Since the thickness of the organic insulating film31 is as large as 500 nm or more, the electric field generated byapplication of a negative voltage to the conductive film 13 functioningas a gate electrode does not affect the surface of the organicinsulating film 31 and the surface of the organic insulating film 31 isnot positively charged easily. Moreover, since the thickness is as largeas 500 nm or more, electric fields of positively charged particlescontained in the air does not affect the interface between the oxidesemiconductor film 19 a and the inorganic insulating film 30 even whenthe positively charged particles are adsorbed on the surface of theorganic insulating film 31. Thus, the interface between the oxidesemiconductor film 19 a and the inorganic insulating film 30 is notsubstantially a state to which a positive bias is applied and thereforethe amount of change in threshold voltage of the transistor is small.

Although water or the like diffuses easily in the organic insulatingfilm 31, water from the outside does not diffuse to a semiconductordevice through the organic insulating film 31 because the organicinsulating film is isolated in each transistor 10. In addition, anitride insulating film is included in the inorganic insulating film 30,whereby water diffused from the outside to the organic insulating film31 can be prevented from diffusing to the oxide semiconductor film 19 a.

As described above, the variation in electrical characteristics of thetransistors can be reduced by providing the isolated organic insulatingfilms 31 over the transistors. In addition, a transistor havingnormally-off characteristic and high reliability can be manufactured.Moreover, the organic insulating film can be formed by a printingmethod, a coating method, or the like; therefore, the manufacturing timecan be shortened.

Modification Example 1

A modification example of the transistor described in this embodiment isdescribed with reference to FIG. 1D. A transistor 10 a shown in thismodification example includes an oxide semiconductor film 19 g and apair of conductive films 21 f and 21 g which are formed with amulti-tone photomask.

With the use of a multi-tone photomask, a resist mask having a pluralityof thicknesses can be formed. After the oxide semiconductor film 19 g isformed with the resist mask, the resist mask is exposed to oxygen plasmaor the like and is partly removed; accordingly, a resist mask forforming a pair of conductive films is formed. Therefore, the number ofsteps in the photolithography process in the process of forming theoxide semiconductor film 19 g and the pair of conductive films 21 f and21 g can be reduced.

Note that outside the pair of conductive films 21 f and 21 g, the oxidesemiconductor film 19 g formed with a multi-tone photomask is partlyexposed in the planar shape.

Modification Example 2

A modification example of the transistor described in this embodiment isdescribed with reference to FIG. 1E. A transistor 10 b described in thismodification example is a channel-protective transistor.

The transistor 10 b illustrated in FIG. 1E includes the conductive film13 functioning as a gate electrode provided over the first substrate 11,the gate insulating film 14 formed over the first substrate 11 and theconductive film 13 functioning as a gate electrode, the oxidesemiconductor film 19 a overlapping with the conductive film 13functioning as a gate electrode with the gate insulating film 14provided therebetween, an inorganic insulating film 30 a covering achannel region and side surfaces of the oxide semiconductor film 19 a,and conductive films 21 h and 21 i functioning as a source electrode anda drain electrode in contact with the oxide semiconductor film 19 a inan opening of the inorganic insulating film 30 a. In addition, theorganic insulating film 31 overlapping with the oxide semiconductor film19 a with the inorganic insulating film 30 a provided therebetween isincluded. The organic insulating film 31 is provided over the conductivefilms 21 h and 21 i and the inorganic insulating film 30 a.

In the channel-protective transistor, the oxide semiconductor film 19 ais not damaged by etching for forming the conductive films 21 h and 21 ibecause the oxide semiconductor film 19 a is covered with the inorganicinsulating film 30 a. Therefore, defects in the oxide semiconductor film19 a can be reduced.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments.

Embodiment 2

In this embodiment, a display device which is one embodiment of thepresent invention is described with reference to drawings.

FIG. 3A illustrates an example of a display device. A display deviceillustrated in FIG. 3A includes a pixel portion 101; a scan line drivercircuit 104; a signal line driver circuit 106; m scan lines 107 whichare arranged parallel or substantially parallel to each other and whosepotentials are controlled by the scan line driver circuit 104; and nsignal lines 109 which are arranged parallel or substantially parallelto each other and whose potentials are controlled by the signal linedriver circuit 106. The pixel portion 101 further includes a pluralityof pixels 103 arranged in a matrix. Furthermore, capacitor lines 115arranged parallel or substantially parallel are provided along thesignal lines 109. Note that the capacitor lines 115 may be arrangedparallel or substantially parallel along the scan lines 107. The scanline driver circuit 104 and the signal line driver circuit 106 arecollectively referred to as a driver circuit portion in some cases.

In addition, the display device also includes a driver circuit fordriving a plurality of pixels, and the like. The display device may alsobe referred to as a liquid crystal module including a control circuit, apower supply circuit, a signal generation circuit, a backlight module,and the like provided over another substrate.

Each scan line 107 is electrically connected to the n pixels 103 in thecorresponding row among the pixels 103 arranged in m rows and n columnsin the pixel portion 101. Each signal line 109 is electrically connectedto the m pixels 103 in the corresponding column among the pixels 103arranged in m rows and n columns. Note that m and n are each an integerof 1 or more. Each capacitor line 115 is electrically connected to the mpixels 103 in the corresponding columns among the pixels 103 arranged inm rows and n columns. Note that in the case where the capacitor lines115 are arranged parallel or substantially parallel along the scan lines107, each capacitor line 115 is electrically connected to the n pixels103 in the corresponding rows among the pixels 103 arranged in m rowsand n columns.

Note that here, a pixel refers to a region surrounded by scan lines andsignal lines and exhibiting one color. Therefore, in the case of a colordisplay device having color elements of R (red), G (green), and B(blue), a minimum unit of an image is composed of three pixels of an Rpixel, a G pixel, and a B pixel. Note that color reproducibility can beimproved by adding a yellow pixel, a cyan pixel, a magenta pixel, or thelike to the R pixel, the G pixel, and the B pixel. Moreover, powerconsumption of the display device can be reduced by adding a W (white)pixel to the R pixel, the G pixel, and the B pixel. In the case of aliquid crystal display device, brightness of the liquid crystal displaydevice can be improved by adding a W pixel to each of the R pixel, the Gpixel, and the B pixel. As a result, the power consumption of the liquidcrystal display device can be reduced.

FIGS. 3B and 3C illustrate examples of a circuit configuration that canbe used for the pixels 103 in the display device illustrated in FIG. 3A.

The pixel 103 in FIG. 3B includes a liquid crystal element 121, atransistor 102, and a capacitor 105.

The potential of one of a pair of electrodes of the liquid crystalelement 121 is set as appropriate according to the specifications of thepixel 103. The alignment state of the liquid crystal element 121 dependson written data. A common potential may be supplied to one of the pairof electrodes of the liquid crystal element 121 included in each of aplurality of pixels 103. Furthermore, the potential supplied to the oneof the pair of electrodes of the liquid crystal element 121 in the pixel103 in one row may be different from the potential supplied to the oneof the pair of electrodes of the liquid crystal element 121 in the pixel103 in another row.

The liquid crystal element 121 is an element that controls transmissionor non-transmission of light utilizing an optical modulation action ofliquid crystal. Note that the optical modulation action of the liquidcrystal is controlled by an electric field applied to the liquid crystal(including a horizontal electric field, a vertical electric field, and adiagonal electric field). Examples of the liquid crystal element 121 area nematic liquid crystal, a cholesteric liquid crystal, a smectic liquidcrystal, a thermotropic liquid crystal, a lyotropic liquid crystal, aferroelectric liquid crystal, and an anti-ferroelectric liquid crystal.

As examples of a driving method of the display device including theliquid crystal element 121, any of the following modes can be given: aTN mode, a VA mode, an ASM (axially symmetric aligned micro-cell) mode,an OCB (optically compensated birefringence) mode, an MVA mode, a PVA(patterned vertical alignment) mode, an IPS mode, an FFS mode, a TBA(transverse bend alignment) mode, and the like. Note that one embodimentof the present invention is not limited to this, and various liquidcrystal elements and driving methods can be used as a liquid crystalelement and a driving method thereof.

The liquid crystal element may be formed using a liquid crystalcomposition including liquid crystal exhibiting a blue phase and achiral material. The liquid crystal exhibiting a blue phase has a shortresponse time of 1 msec or less and is optically isotropic; therefore,alignment treatment is not necessary and viewing angle dependence issmall.

In the structure of the pixel 103 illustrated in FIG. 3B, one of asource electrode and a drain electrode of the transistor 102 iselectrically connected to the signal line 109, and the other iselectrically connected to the other of the pair of electrodes of theliquid crystal element 121. A gate electrode of the transistor 102 iselectrically connected to the scan line 107. The transistor 102 has afunction of controlling whether to write a data signal by being turnedon or off.

In the pixel 103 in FIG. 3B, one of a pair of electrodes of thecapacitor 105 is electrically connected to the capacitor line 115 towhich a potential is supplied, and the other thereof is electricallyconnected to the other of the pair of electrodes of the liquid crystalelement 121. The potential of the capacitor line 115 is set asappropriate in accordance with the specifications of the pixel 103. Thecapacitor 105 functions as a storage capacitor for storing written data.

For example, in the display device including the pixel 103 in FIG. 3B,the pixels 103 are sequentially selected row by row by the scan linedriver circuit 104, whereby the transistors 102 are turned on and dataof a data signal is written.

When the transistors 102 are turned off, the pixels 103 in which thedata has been written are brought into a holding state. This operationis sequentially performed row by row; thus, an image is displayed.

The pixel 103 in FIG. 3C includes a transistor 133 performing switchingof a display element, the transistor 102 controlling pixel driving, atransistor 135, the capacitor 105, and a light-emitting element 131.

One of a source electrode and a drain electrode of the transistor 133 iselectrically connected to the signal line 109 to which a data signal issupplied. A gate electrode of the transistor 133 is electricallyconnected to a scan line 107 to which a gate signal is supplied.

The transistor 133 has a function of controlling whether to write a datasignal by being turned on or off.

One of a source electrode and a drain electrode of the transistor 102 iselectrically connected to a wiring 137 serving as an anode line, and theother is electrically connected to one electrode of the light-emittingelement 131. The gate electrode of the transistor 102 is electricallyconnected to the other of the source electrode and the drain electrodeof the transistor 133 and one electrode of the capacitor 105.

The transistor 102 has a function of controlling current flowing throughthe light-emitting element 131 by being turned on or off.

One of a source electrode and a drain electrode of the transistor 135 isconnected to a wiring 139 to which a reference potential of data issupplied, and the other thereof is electrically connected to the oneelectrode of the light-emitting element 131 and the other electrode ofthe capacitor 105. Moreover, a gate electrode of the transistor 135 iselectrically connected to the scan line 107 to which the gate signal issupplied.

The transistor 135 has a function of adjusting the current flowingthrough the light-emitting element 131. For example, when the internalresistance of the light-emitting element 131 increases because ofdeterioration or the like of the light-emitting element 131, the currentflowing through the light-emitting element 131 can be corrected bymonitoring current flowing through the wiring 139 to which the one ofthe source electrode and the drain electrode of the transistor 135 isconnected. The potential supplied to the wiring 139 can be set to 0 V,for example.

The one electrode of the capacitor 105 is electrically connected to thegate electrode of the transistor 102 and the other of the sourceelectrode and the drain electrode of the transistor 133, and the otherelectrode of the capacitor 105 is electrically connected to the other ofthe source electrode and the drain electrode of the transistor 135 andthe one electrode of the light-emitting element 131.

In the pixel 103 in FIG. 3C, the capacitor 105 functions as a storagecapacitor for storing written data.

The one electrode of the light-emitting element 131 is electricallyconnected to the other of the source electrode and the drain electrodeof the transistor 135, the other electrode of the capacitor 105, and theother of the source electrode and the drain electrode of the transistor102. Furthermore, the other electrode of the light-emitting element 131is electrically connected to a wiring 141 serving as a cathode.

As the light-emitting element 131, an organic electroluminescent element(also referred to as an organic EL element) or the like can be used, forexample. Note that the light-emitting element 131 is not limited to anorganic EL element; an inorganic EL element including an inorganicmaterial may be used.

A high power supply potential VDD is supplied to one of the wiring 137and the wiring 141, and a low power supply potential VSS is supplied tothe other. In the structure of FIG. 3C, the high power supply potentialVDD is supplied to the wiring 137, and the low power supply potentialVSS is supplied to the wiring 141.

For example, in the display device including the pixel 103 in FIG. 3C,the pixels 103 are sequentially selected row by row by the scan linedriver circuit 104, whereby the transistors 102 are turned on and dataof a data signal is written.

When the transistors 133 are turned off, the pixels 103 in which thedata has been written are brought into a holding state. The transistor133 is connected to the capacitor 105, and thus written data can bestored for a long period. The amount of current flowing between thesource and drain electrodes is controlled by the transistor 133. Thelight-emitting element 131 emits light with a luminance corresponding tothe amount of flowing current. This operation is sequentially performedrow by row; thus, an image is displayed.

Note that although FIGS. 3B and 3C each illustrate an example where theliquid crystal element 121 and the light-emitting element 131 are usedas a display element, one embodiment of the present invention is notlimited thereto. Any of a variety of display elements may be used.Examples of display elements include elements including a display mediumwhose contrast, luminance, reflectance, transmittance, or the like ischanged by electromagnetic action, such as an EL (electroluminescent)element (e.g., an EL element including organic and inorganic materials,an organic EL element, and an inorganic EL element), an LED (e.g., awhite LED, a red LED, a green LED, and a blue LED), a transistor (atransistor that emits light depending on a current), an electronemitter, electronic ink, an electrophoretic element, a grating lightvalve (GLV), a plasma display panel (PDP), a micro electro mechanicalsystem (MEMS), a digital micromirror device (DMD), a digital microshutter (DMS), an interferometric modulator display (IMOD), anelectrowetting element, a piezoelectric ceramic display, and a carbonnanotube. Note that examples of display devices including EL elementsinclude an EL display. Examples of display devices including electronemitters are a field emission display (FED) and an SED-type flat paneldisplay (SED: surface-conduction electron-emitter display). Examples ofdisplay devices including liquid crystal elements include a liquidcrystal display (e.g., a transmissive liquid crystal display, atransflective liquid crystal display, a reflective liquid crystaldisplay, a direct-view liquid crystal display, or a projection liquidcrystal display) and the like. An example of a display device includingelectronic ink or electrophoretic elements is electronic paper.

Next, a specific structure of an element substrate included in thedisplay device is described. Here, a specific example of a liquidcrystal display device including a liquid crystal element in the pixel103 is described. FIG. 4 is a top view of the pixel 103 shown in FIG.3B.

Here, a liquid crystal display device driven in an FFS mode is used asthe display device, and FIG. 4 is a top view of a plurality of pixels103 a, 103 b, and 103 c included in the liquid crystal display device.

In FIG. 4, a conductive film 13 functioning as a scan line extends in adirection substantially perpendicularly to a conductive film functioningas a signal line (in the lateral direction in the drawing). Theconductive film 21 a functioning as a signal line extend in a directionsubstantially perpendicularly to the conductive film functioning as ascan line (in the vertical direction in the drawing). Note that theconductive film 13 functioning as a scan line is electrically connectedto the scan line driver circuit 104 (see FIG. 3A), and the conductivefilm 21 a functioning as a signal line is electrically connected to thesignal line driver circuit 106 (see FIG. 3A).

The transistor 102 is provided in a region where the conductive filmfunctioning as a scan line and the conductive film functioning as asignal line intersect with each other. The transistor 102 includes theconductive film 13 functioning as a gate electrode; a gate insulatingfilm (not illustrated in FIG. 4); the oxide semiconductor film 19 awhere a channel region is formed, over the gate insulating film; and theconductive film 21 a and a conductive film 21 b functioning as a sourceelectrode and a drain electrode. The conductive film 13 also functionsas a conductive film functioning as a scan line, and a region of theconductive film 13 that overlaps with the oxide semiconductor film 19 aserves as the gate electrode of the transistor 102. In addition, theconductive film 21 a also functions as a conductive film functioning asa signal line, and a region of the conductive film 21 a that overlapswith the oxide semiconductor film 19 a functions as the source electrodeor the drain electrode of the transistor 102. Furthermore, in the topview of FIG. 4, an end portion of the conductive film functioning as ascan line is positioned on an outer side of an end portion of the oxidesemiconductor film 19 a. Thus, the conductive film functioning as a scanline functions as a light-blocking film for blocking light from a lightsource such as a backlight. For this reason, the oxide semiconductorfilm 19 a included in the transistor is not irradiated with light, sothat a variation in the electrical characteristics of the transistor canbe suppressed.

In addition, the transistor 102 includes the organic insulating film 31overlapping with the oxide semiconductor film 19 a. The organicinsulating film 31 overlaps with the oxide semiconductor film 19 a (inparticular, a region of the oxide semiconductor film 19 a which isbetween the conductive films 21 a and 21 b) with an inorganic insulatingfilm (not illustrated in FIG. 4) provided therebetween.

Water from the outside does not diffuse to the liquid crystal displaydevice through the organic insulating film 31 because the organicinsulating film 31 is isolated in each transistor 10; therefore, thevariation in electrical characteristics of the transistors provided inthe liquid crystal display device can be reduced.

The conductive film 21 b is electrically connected to a pixel electrode19 b. A common electrode 29 is provided over the pixel electrode 19 bwith an insulating film provided therebetween. An opening 40 indicatedby a dashed-dotted line is provided in the insulating film provided overthe pixel electrode 19 b. The pixel electrode 19 b is in contact with anitride insulating film (not illustrated in FIG. 4) in the opening 40.

The common electrode 29 includes stripe regions extending in a directionintersecting with the conductive film 21 a functioning as a signal line.The stripe regions are connected to a region extending in a directionparallel or substantially parallel to the conductive film 21 afunctioning as a signal line. Accordingly, the stripe regions of thecommon electrode 29 are at the same potential in pixels.

The capacitor 105 is formed in a region where the pixel electrode 19 band the common electrode 29 overlap with each other. The pixel electrode19 b and the common electrode 29 each have a light-transmittingproperty. That is, the capacitor 105 has a light-transmitting property.

As illustrated in FIG. 4, an FFS mode liquid crystal display device isprovided with the common electrode including the stripe regionsextending in a direction intersecting with the conductive filmfunctioning as a signal line. Thus, the display device can haveexcellent contrast.

Owing to the light-transmitting property of the capacitor 105, thecapacitor 105 can be formed large (in a large area) in the pixel 103.Thus, a display device with a large-capacitance capacitor as well as anaperture ratio increased to typically 50% or more, preferably 60% ormore can be provided. For example, in a high-resolution display devicesuch as a liquid crystal display device, the area of a pixel is smalland accordingly the area of a capacitor is also small. For this reason,the amount of charges accumulated in the capacitor is small in thehigh-resolution display device. However, since the capacitor 105 of thisembodiment has a light-transmitting property, when the capacitor 105 isprovided in a pixel, sufficient capacitance can be obtained in the pixeland the aperture ratio can be improved. Typically, the capacitor 105 canbe favorably used for a high-resolution display device with a pixeldensity of 200 pixels per inch (ppi) or more, 300 ppi or more, orfurthermore, 500 ppi or more.

In a liquid crystal display device, as the capacitance value of acapacitor is increased, a period during which the alignment of liquidcrystal molecules of a liquid crystal element can be kept constant inthe state where an electric field is applied can be made longer. Whenthe period can be made longer in a display device which displays a stillimage, the number of times of rewriting image data can be reduced,leading to a reduction in power consumption. Furthermore, according tothe structure of this embodiment, the aperture ratio can be improvedeven in a high-resolution display device, which makes it possible to uselight from a light source such as a backlight efficiently, so that powerconsumption of the display device can be reduced.

Next, FIG. 5 is a cross-sectional view taken along dashed-dotted linesA-B and C-D in FIG. 4. The transistor 102 illustrated in FIG. 5 is achannel-etched transistor. Note that the transistor 102 in the channellength direction and the capacitor 105 are illustrated in thecross-sectional view taken along dashed-dotted line A-B, and thetransistor 102 in the channel width direction is illustrated in thecross-sectional view taken along dashed-dotted line C-D.

The liquid crystal display device described in this embodiment includesa pair of substrates (the first substrate 11 and a second substrate342), an element layer in contact with the first substrate 11, anelement layer in contact with the second substrate 342, and a liquidcrystal element 320 provided between the element layers. Note that theelement layer is a generic term used to refer to layers interposedbetween the substrate and the liquid crystal layer. A liquid crystalelement 322 is provided between a pair of substrates (the firstsubstrate 11 and the second substrate 342).

The liquid crystal element 322 includes the pixel electrode 19 b overthe first substrate 11, the common electrode 29, a nitride insulatingfilm 27, a film controlling alignment (hereinafter referred to as analignment film 33), and the liquid crystal layer 320. The pixelelectrode 19 b functions as one electrode of the liquid crystal element322, and the common electrode 29 functions as the other electrode of theliquid crystal element 322.

First, the element layer formed over the first substrate 11 isdescribed. The transistor 102 in FIG. 5 has a single-gate structure andincludes the conductive film 13 functioning as a gate electrode over thefirst substrate 11. In addition, the transistor 102 includes a nitrideinsulating film 15 formed over the first substrate 11 and the conductivefilm 13 functioning as a gate electrode, an oxide insulating film 17formed over the nitride insulating film 15, the oxide semiconductor film19 a overlapping with the conductive film 13 functioning as a gateelectrode with the nitride insulating film 15 and the oxide insulatingfilm 17 provided therebetween, and the conductive films 21 a and 21 bfunctioning as a source electrode and a drain electrode which are incontact with the oxide semiconductor film 19 a. The nitride insulatingfilm 15 and the oxide insulating film 17 function as the gate insulatingfilm 14. Moreover, an oxide insulating film 23 is formed over the oxideinsulating film 17, the oxide semiconductor film 19 a, and theconductive films 21 a and 21 b functioning as a source electrode and adrain electrode, and an oxide insulating film 25 is formed over theoxide insulating film 23. The nitride insulating film 27 is formed overthe oxide insulating film 23, the oxide insulating film 25, and theconductive film 21 b. The oxide insulating film 23, the oxide insulatingfilm 25, and the nitride insulating film 27 function as the inorganicinsulating film 30. The pixel electrode 19 b is formed over the oxideinsulating film 17. The pixel electrode 19 b is connected to one of theconductive films 21 a and 21 b functioning as a source electrode and adrain electrode, here, connected to the conductive film 21 b. The commonelectrode 29 is formed over the nitride insulating film 27. In addition,the organic insulating film 31 overlapping with the oxide semiconductorfilm 19 a of the transistor 102 with the inorganic insulating film 30provided therebetween is included.

A region where the pixel electrode 19 b, the nitride insulating film 27,and the common electrode 29 overlap with one another functions as thecapacitor 105.

The thickness of the organic insulating film 31 is preferably greaterthan or equal to 500 nm and less than or equal to 10 μm. The thicknessof the organic insulating film 31 in FIG. 5 is smaller than a gapbetween the inorganic insulating film 30 formed over the first substrate11 and the element layer formed on the second substrate 342. Therefore,the liquid crystal layer 320 is provided between the organic insulatingfilm 31 and the element layer formed on the second substrate 342. Inother words, the liquid crystal layer 320 is provided between thealignment film 33 over the organic insulating film 31 and an alignmentfilm 352 included in the element layer on the second substrate 342.

Note that as illustrated in FIG. 6, the alignment film 33 over theorganic insulating film 31 a and the alignment film 352 included in theelement layer on the second substrate 342 may be in contact with eachother. In this case, the organic insulating film 31 a functions as aspacer; therefore, the cell gap of the liquid crystal display device canbe maintained with the organic insulating film 31 a.

Although the alignment film 33 is provided over the organic insulatingfilm 31, the organic insulating film 31 a, or the like in FIG. 5, FIG.6, or the like, one embodiment of the present invention is not limitedthereto. As illustrated in FIG. 33, an organic insulating film 31 b maybe provided over the alignment film 33 in some cases or depending oncircumstances. In this case, a rubbing step may be performed after theformation of the organic insulating film 31 b or the like over thealignment film 33 instead of directly after the formation of thealignment film 33, for example.

The organic insulating film 31, 31 a, or 31 b overlapping with the oxidesemiconductor film 19 a is provided over the transistor 102, whereby thesurface of the oxide semiconductor film 19 a can be made apart from thesurface of the organic insulating film 31, 31 a, or 31 b. Thus, thesurface of the oxide semiconductor film 19 a is not affected by theelectric field of positively charged particles adsorbed on the surfaceof the organic insulating film 31, 31 a, or 31 b and therefore thereliability of the transistor 102 can be improved.

Note that a cross-sectional view of one embodiment of the presentinvention is not limited to FIG. 5, FIG. 6, and FIG. 33. The displaydevice can have a variety of different structures. For example, thepixel electrode 19 b may have a slit. The pixel electrode 19 b may havea comb-like shape. An example of a cross-sectional view in this case isshown in FIG. 7. Alternatively, an organic insulating film 31 c which isnot isolated may be provided over the nitride insulating film 27 asillustrated in FIG. 8. For example, the organic insulating film 31 cwhich is not isolated is provided, whereby the surface of the organicinsulating film 31 c can be made flat. In other words, as an example,the organic insulating film 31 c can function as a planarization film.Alternatively, a capacitor 105 b may be formed so that the commonelectrode 29 and the conductive film 21 b overlap with each other asillustrated in FIG. 9. Such a structure enables the capacitor 105 b tofunction as a capacitor holding the potential of the pixel electrode.Therefore, with such a structure, capacitance of the capacitor can beincreased.

A structure of the display device is described below in detail.

There is no particular limitation on the property of a material and thelike of the first substrate 11 as long as the material has heatresistance enough to withstand at least later heat treatment. Forexample, a glass substrate, a ceramic substrate, a quartz substrate, ora sapphire substrate may be used as the first substrate 11.Alternatively, a single crystal semiconductor substrate or apolycrystalline semiconductor substrate made of silicon, siliconcarbide, or the like, a compound semiconductor substrate made of silicongermanium or the like, an SOI (silicon on insulator) substrate, or thelike may be used as the first substrate 11. Furthermore, any of thesesubstrates further provided with a semiconductor element may be used asthe first substrate 11. In the case where a glass substrate is used asthe first substrate 11, a glass substrate having any of the followingsizes can be used: the 6th generation (1500 mm×1850 mm), the 7thgeneration (1870 mm×2200 mm), the 8th generation (2200 mm×2400 mm), the9th generation (2400 mm×2800 mm), and the 10th generation (2950 mm×3400mm). Thus, a large-sized display device can be manufactured.

Alternatively, a flexible substrate may be used as the first substrate11, and the transistor 102 may be provided directly on the flexiblesubstrate. Alternatively, a separation layer may be provided between thefirst substrate 11 and the transistor 102. The separation layer can beused when part or the whole of a display device formed over theseparation layer is separated from the first substrate 11 andtransferred onto another substrate. In such a case, the transistor 102can be transferred to a substrate having low heat resistance or aflexible substrate as well.

The conductive film 13 functioning as a gate electrode can be formedusing a metal element selected from aluminum, chromium, copper,tantalum, titanium, molybdenum, and tungsten; an alloy containing any ofthese metal elements as a component; an alloy containing any of thesemetal elements in combination; or the like. Furthermore, one or moremetal elements selected from manganese and zirconium may be used. Theconductive film 13 functioning as a gate electrode may have asingle-layer structure or a stacked-layer structure of two or morelayers. For example, a single-layer structure of an aluminum filmcontaining silicon, a two-layer structure in which an aluminum film isstacked over a titanium film, a two-layer structure in which a titaniumfilm is stacked over a titanium nitride film, a two-layer structure inwhich a tungsten film is stacked over a titanium nitride film, atwo-layer structure in which a tungsten film is stacked over a tantalumnitride film or a tungsten nitride film, a two-layer structure in whicha copper film is stacked over a titanium film, a two-layer structure inwhich a copper film is stacked over a molybdenum film, and a three-layerstructure in which a titanium film, an aluminum film, and a titaniumfilm are stacked in this order can be given. Alternatively, an alloyfilm or a nitride film which contains aluminum and one or more elementsselected from titanium, tantalum, tungsten, molybdenum, chromium,neodymium, and scandium may be used.

The conductive film 13 functioning as a gate electrode can also beformed using a light-transmitting conductive material such as indium tinoxide, indium oxide containing tungsten oxide, indium zinc oxidecontaining tungsten oxide, indium oxide containing titanium oxide,indium tin oxide containing titanium oxide, indium zinc oxide, or indiumtin oxide to which silicon oxide is added. It is also possible to have astacked-layer structure formed using the above light-transmittingconductive material and the above metal element.

The nitride insulating film 15 can be a nitride insulating film that ishardly permeated by oxygen. Furthermore, a nitride insulating film whichis hardly permeated by oxygen, hydrogen, and water can be used. As thenitride insulating film that is hardly permeated by oxygen and thenitride insulating film that is hardly permeated by oxygen, hydrogen,and water, a silicon nitride film, a silicon nitride oxide film, analuminum nitride film, an aluminum nitride oxide film, or the like isgiven. Instead of the nitride insulating film that is hardly permeatedby oxygen and the nitride insulating film that is hardly permeated byoxygen, hydrogen, and water, an oxide insulating film such as analuminum oxide film, an aluminum oxynitride film, a gallium oxide film,a gallium oxynitride film, an yttrium oxide film, an yttrium oxynitridefilm, a hafnium oxide film, or a hafnium oxynitride film can be used.

The thickness of the nitride insulating film 15 is preferably greaterthan or equal to 5 nm and less than or equal to 100 nm, furtherpreferably greater than or equal to 20 nm and less than or equal to 80nm.

The oxide insulating film 17 may be formed to have a single-layerstructure or a stacked-layer structure using, for example, one or moreof a silicon oxide film, a silicon oxynitride film, a silicon nitrideoxide film, a silicon nitride film, an aluminum oxide film, a hafniumoxide film, a gallium oxide film, a Ga—Zn-based metal oxide film, and asilicon nitride film.

The oxide insulating film 17 may also be formed using a material havinga high relative dielectric constant such as hafnium silicate(HfSiO_(x)), hafnium silicate to which nitrogen is added(HfSi_(x)O_(y)N_(z)), hafnium aluminate to which nitrogen is added(HfAl_(x)O_(y)N_(z)), hafnium oxide, or yttrium oxide, so that gateleakage current of the transistor can be reduced.

The thickness of the oxide insulating film 17 is preferably greater thanor equal to 5 nm and less than or equal to 400 nm, further preferablygreater than or equal to 10 nm and less than or equal to 300 nm, stillfurther preferably greater than or equal to 50 nm and less than or equalto 250 nm.

The oxide semiconductor film 19 a is typically formed using an In—Gaoxide, an In—Zn oxide, or an In-M-Zn oxide (M represents Al, Ga, Y, Zr,Sn, La, Ce, or Nd).

In the case where the oxide semiconductor film 19 a contains an In-M-Znoxide, the proportions of In and M when summation of In and M is assumedto be 100 atomic % are preferably as follows: the atomic percentage ofIn is greater than 25 atomic % and the atomic percentage of M is lessthan 75 atomic %, or further preferably, the atomic percentage of In isgreater than 34 atomic % and the atomic percentage of M is less than 66atomic %.

The energy gap of the oxide semiconductor film 19 a is 2 eV or more,preferably 2.5 eV or more, further preferably 3 eV or more. Theoff-state current of the transistor 102 can be reduced by using an oxidesemiconductor having such a wide energy gap.

The thickness of the oxide semiconductor film 19 a is greater than orequal to 3 nm and less than or equal to 200 nm, preferably greater thanor equal to 3 nm and less than or equal to 100 nm, further preferablygreater than or equal to 3 nm and less than or equal to 50 nm.

In the case where the oxide semiconductor film 19 a is an In-M-Zn oxidefilm (M represents Al, Ga, Y, Zr, Sn, La, Ce, or Nd), it is preferablethat the atomic ratio of metal elements of a sputtering target used forforming the In-M-Zn oxide film satisfy In M and Zn M. As the atomicratio of metal elements of such a sputtering target, In:M:Zn=1:1:1,In:M:Zn=1:1:1.2, and In:M:Zn=3:1:2 are preferable. Note that theproportion of each metal element in the atomic ratio of the oxidesemiconductor film 19 a to be formed varies within a range of ±40% ofthat in the above atomic ratio of the sputtering target as an error.

An oxide semiconductor film with low carrier density is used as theoxide semiconductor film 19 a. For example, an oxide semiconductor filmwhose carrier density is 1×10¹⁷/cm³ or lower, preferably 1×10¹⁵/cm³ orlower, further preferably 1×10¹³/cm³ or lower, still further preferably1×10¹¹/cm³ or lower is used as the oxide semiconductor film 19 a.

Note that, without limitation to the compositions described above, amaterial with an appropriate composition may be used depending onrequired semiconductor characteristics and electrical characteristics(e.g., field-effect mobility and threshold voltage) of a transistor.Furthermore, in order to obtain required semiconductor characteristicsof a transistor, it is preferable that the carrier density, the impurityconcentration, the defect density, the atomic ratio of a metal elementto oxygen, the interatomic distance, the density, and the like of theoxide semiconductor film 19 a be set to be appropriate.

Note that it is preferable to use, as the oxide semiconductor film 19 a,an oxide semiconductor film in which the impurity concentration is lowand density of defect states is low, in which case the transistor canhave more excellent electrical characteristics. Here, the state in whichimpurity concentration is low and density of defect states is low (theamount of oxygen vacancies is small) is referred to as “highly purifiedintrinsic” or “substantially highly purified intrinsic”. A highlypurified intrinsic or substantially highly purified intrinsic oxidesemiconductor has few carrier generation sources, and thus has a lowcarrier density in some cases. Thus, a transistor in which a channelregion is formed in the oxide semiconductor film rarely has a negativethreshold voltage (is rarely normally on). A highly purified intrinsicor substantially highly purified intrinsic oxide semiconductor film hasa low density of defect states and accordingly has few carrier traps insome cases. Furthermore, the highly purified intrinsic or substantiallyhighly purified intrinsic oxide semiconductor film has an extremely lowoff-state current; even when an element has a channel width of 1×10⁶ μmand a channel length (L) of 10 μm, the off-state current can be lessthan or equal to the measurement limit of a semiconductor parameteranalyzer, i.e., less than or equal to 1×10⁻¹³ A, at a voltage (drainvoltage) between a source electrode and a drain electrode of from 1 V to10 V. Thus, the transistor in which a channel region is formed in theoxide semiconductor film has a small variation in electricalcharacteristics and high reliability in some cases. As examples of theimpurities, hydrogen, nitrogen, alkali metal, and alkaline earth metalare given.

Hydrogen contained in the oxide semiconductor film reacts with oxygenbonded to a metal atom to be water, and in addition, an oxygen vacancyis formed in a lattice from which oxygen is released (or a portion fromwhich oxygen is released). Due to entry of hydrogen into the oxygenvacancy, an electron serving as a carrier is generated in some cases.Furthermore, in some cases, bonding of part of hydrogen to oxygen bondedto a metal atom causes generation of an electron serving as a carrier.Thus, a transistor including an oxide semiconductor which containshydrogen is likely to be normally on.

Accordingly, it is preferable that hydrogen be reduced as much aspossible as well as the oxygen vacancies in the oxide semiconductor film19 a. Specifically, in the oxide semiconductor film 19 a, theconcentration of hydrogen which is measured by secondary ion massspectrometry (SIMS) is set to be lower than or equal to 5×10¹⁹atoms/cm³, preferably lower than or equal to 1×10¹⁹ atoms/cm³, furtherpreferably lower than 5×10¹⁸ atoms/cm³, still further preferably lowerthan or equal to 1×10¹⁸ atoms/cm³, yet still further preferably lowerthan or equal to 5×10¹⁷ atoms/cm³, yet still furthermore preferablylower than or equal to 1×10¹⁶ atoms/cm³.

When silicon or carbon which is one of elements belonging to Group 14 iscontained in the oxide semiconductor film 19 a, oxygen vacancies areincreased, and the oxide semiconductor film 19 a becomes an n-type film.Thus, the concentration of silicon or carbon (the concentration ismeasured by SIMS) of the oxide semiconductor film 19 a is set to belower than or equal to 2×10¹⁸ atoms/cm³, preferably lower than or equalto 2×10¹⁷ atoms/cm³.

The concentration of alkali metal or alkaline earth metal in the oxidesemiconductor film 19 a, which is measured by SIMS, is set to be lowerthan or equal to 1×10¹⁸ atoms/cm³, preferably lower than or equal to2×10¹⁶ atoms/cm³. Alkali metal and alkaline earth metal might generatecarriers when bonded to an oxide semiconductor, in which case theoff-state current of the transistor might be increased. Therefore, it ispreferable to reduce the concentration of alkali metal or alkaline earthmetal in the oxide semiconductor film 19 a.

Furthermore, when containing nitrogen, the oxide semiconductor film 19 aeasily has n-type conductivity by generation of electrons serving ascarriers and an increase of carrier density. Thus, a transistorincluding an oxide semiconductor which contains nitrogen is likely to benormally on. For this reason, nitrogen in the oxide semiconductor filmis preferably reduced as much as possible; the concentration of nitrogenwhich is measured by SIMS is preferably set to be, for example, lowerthan or equal to 5×10¹⁸ atoms/cm³.

The oxide semiconductor film 19 a may have a non-single-crystalstructure, for example. The non-single-crystal structure includes ac-axis aligned crystalline oxide semiconductor (CAAC-OS) which isdescribed later, a polycrystalline structure, a microcrystallinestructure which is described later, or an amorphous structure, forexample. Among the non-single-crystal structures, the amorphousstructure has the highest density of defect states, whereas CAAC-OS hasthe lowest density of defect states.

The oxide semiconductor film 19 a may have a non-single-crystalstructure, for example. The oxide semiconductor films having theamorphous structure each have disordered atomic arrangement and nocrystalline component, for example.

Note that the oxide semiconductor film 19 a may be a mixed filmincluding two or more regions of the following: a region having anamorphous structure, a region having a microcrystalline structure, aregion having a polycrystalline structure, a CAAC-OS region, and aregion having a single-crystal structure. The mixed film has asingle-layer structure including, for example, two or more of a regionhaving an amorphous structure, a region having a microcrystallinestructure, a region having a polycrystalline structure, a CAAC-OSregion, and a region having a single-crystal structure in some cases.Furthermore, the mixed film has a stacked-layer structure of two or moreof a region having an amorphous structure, a region having amicrocrystalline structure, a region having a polycrystalline structure,a CAAC-OS region, and a region having a single-crystal structure in somecases.

The pixel electrode 19 b is formed by processing an oxide semiconductorfilm formed at the same time as the oxide semiconductor film 19 a. Thus,the pixel electrode 19 b contains a metal element similar to that in theoxide semiconductor film 19 a. Furthermore, the pixel electrode 19 b hasa crystal structure similar to or different from that of the oxidesemiconductor film 19 a. By adding impurities or oxygen vacancies to theoxide semiconductor film formed at the same time as the oxidesemiconductor film 19 a, the oxide semiconductor film has conductivityand thus functions as the pixel electrode 19 b. An example of theimpurities contained in the oxide semiconductor film is hydrogen.Instead of hydrogen, as the impurity, boron, phosphorus, tin, antimony,a rare gas element, an alkali metal, an alkaline earth metal, or thelike may be included. Alternatively, the pixel electrode 19 b is formedat the same time as the oxide semiconductor film 19 a, and has increasedconductivity by containing oxygen vacancies generated by plasma damageor the like. Alternatively, the pixel electrode 19 b is formed at thesame time as the oxide semiconductor film 19 a, and has increasedconductivity by containing impurities and oxygen vacancies generated byplasma damage or the like.

The oxide semiconductor film 19 a and the pixel electrode 19 b are bothformed over the oxide insulating film 17, but differ in impurityconcentration. Specifically, the pixel electrode 19 b has a higherimpurity concentration than the oxide semiconductor film 19 a. Forexample, the concentration of hydrogen contained in the oxidesemiconductor film 19 a is lower than or equal to 5×10¹⁹ atoms/cm³,preferably lower than or equal to 5×10¹⁸ atoms/cm³, further preferablylower than or equal to 1×10¹⁸ atoms/cm³, still further preferably lowerthan or equal to 5×10¹⁷ atoms/cm³, yet further preferably lower than orequal to 1×10¹⁶ atoms/cm³. The concentration of hydrogen contained inthe pixel electrode 19 b is higher than or equal to 8×10¹⁹ atoms/cm³,preferably higher than or equal to 1×10²⁰ atoms/cm³, further preferablyhigher than or equal to 5×10²⁰ atoms/cm³. The concentration of hydrogencontained in the pixel electrode 19 b is greater than or equal to 2times, preferably greater than or equal to 10 times that in the oxidesemiconductor film 19 a.

When the oxide semiconductor film formed at the same time as the oxidesemiconductor film 19 a is exposed to plasma, the oxide semiconductorfilm is damaged, and oxygen vacancies can be generated. For example,when a film is formed over the oxide semiconductor film by a plasma CVDmethod or a sputtering method, the oxide semiconductor film is exposedto plasma and oxygen vacancies are generated. Alternatively, when theoxide semiconductor film is exposed to plasma in etching treatment forformation of the oxide insulating film 23 and the oxide insulating film25, oxygen vacancies are generated. Alternatively, when the oxidesemiconductor film is exposed to plasma of a mixed gas of oxygen andhydrogen, hydrogen, a rare gas, ammonia, and the like, oxygen vacanciesare generated. As a result, the conductivity of the oxide semiconductorfilm is increased, so that the oxide semiconductor film has conductivityand functions as the pixel electrode 19 b.

In other words, the pixel electrode 19 b is formed using an oxidesemiconductor film having high conductivity. It can also be said thatthe pixel electrode 19 b is formed using a metal oxide film having highconductivity.

In the case where a silicon nitride film is used as the nitrideinsulating film 27, the silicon nitride film contains hydrogen. Whenhydrogen in the nitride insulating film 27 is diffused to the oxidesemiconductor film formed at the same time as the oxide semiconductorfilm 19 a, hydrogen is bonded to oxygen and electrons serving ascarriers are generated in the oxide semiconductor film. When the siliconnitride film is formed by a plasma CVD method or a sputtering method,the oxide semiconductor film is exposed to plasma and oxygen vacanciesare generated in the oxide semiconductor film. When hydrogen containedin the silicon nitride film enters the oxygen vacancies, electronsserving as carriers are generated. As a result, the conductivity of theoxide semiconductor film is increased, so that the oxide semiconductorfilm functions as the pixel electrode 19 b.

When hydrogen is added to an oxide semiconductor including oxygenvacancies, hydrogen enters oxygen vacant sites and forms a donor levelin the vicinity of the conduction band. As a result, the conductivity ofthe oxide semiconductor is increased, so that the oxide semiconductorbecomes a conductor. An oxide semiconductor having become a conductorcan be referred to as an oxide conductor. In other words, the pixelelectrode 19 b is formed using an oxide conductor film. Oxidesemiconductors generally have a visible light transmitting propertybecause of their large energy gap. An oxide conductor is an oxidesemiconductor having a donor level in the vicinity of the conductionband. Therefore, the influence of absorption due to the donor level issmall, and an oxide conductor has a visible light transmitting propertycomparable to that of an oxide semiconductor.

The pixel electrode 19 b has lower resistivity than the oxidesemiconductor film 19 a. The resistivity of the pixel electrode 19 b ispreferably greater than or equal to 1×10⁻⁸ times and less than 1×10⁻¹times the resistivity of the oxide semiconductor film 19 a. Theresistivity of the pixel electrode 19 b is typically greater than orequal to 1×10⁻³ Ωcm and less than 1×10⁴ Ωcm, preferably greater than orequal to 1×10⁻³ Ωcm and less than 1×10⁻¹ Ωcm.

The conductive films 21 a and 21 b functioning as a source electrode anda drain electrode are each formed to have a single-layer structure or astacked-layer structure including any of metals such as aluminum,titanium, chromium, nickel, copper, yttrium, zirconium, molybdenum,silver, tantalum, and tungsten or an alloy containing any of thesemetals as its main component. For example, a single-layer structure ofan aluminum film containing silicon, a two-layer structure in which analuminum film is stacked over a titanium film, a two-layer structure inwhich an aluminum film is stacked over a tungsten film, a two-layerstructure in which a copper film is stacked over acopper-magnesium-aluminum alloy film, a two-layer structure in which acopper film is stacked over a titanium film, a two-layer structure inwhich a copper film is stacked over a tungsten film, a three-layerstructure in which a titanium film or a titanium nitride film, analuminum film or a copper film, and a titanium film or a titaniumnitride film are stacked in this order, and a three-layer structure inwhich a molybdenum film or a molybdenum nitride film, an aluminum filmor a copper film, and a molybdenum film or a molybdenum nitride film arestacked in this order can be given. Note that a transparent conductivematerial containing indium oxide, tin oxide, or zinc oxide may be used.

As the oxide insulating film 23 or the oxide insulating film 25, anoxide insulating film which contains more oxygen than that in thestoichiometric composition is preferably used. Here, as the oxideinsulating film 23, an oxide insulating film which permeates oxygen isformed, and as the oxide insulating film 25, an oxide insulating filmwhich contains more oxygen than that in the stoichiometric compositionis formed.

The oxide insulating film 23 is an oxide insulating film through whichoxygen is permeated. Thus, oxygen released from the oxide insulatingfilm 25 provided over the oxide insulating film 23 can be moved to theoxide semiconductor film 19 a through the oxide insulating film 23.Moreover, the oxide insulating film 23 also serves as a film whichrelieves damage to the oxide semiconductor film 19 a at the time offorming the oxide insulating film 25 later.

A silicon oxide film, a silicon oxynitride film, or the like with athickness greater than or equal to 5 nm and less than or equal to 150nm, preferably greater than or equal to 5 nm and less than or equal to50 nm can be used as the oxide insulating film 23. Note that in thisspecification, “silicon oxynitride film” refers to a film that containsmore oxygen than nitrogen, and “silicon nitride oxide film” refers to afilm that contains more nitrogen than oxygen.

Furthermore, the oxide insulating film 23 is preferably an oxideinsulating film containing nitrogen and having a small number ofdefects.

Typical examples of the oxide insulating film containing nitrogen andhaving a small number of defects include a silicon oxynitride film andan aluminum oxynitride film.

In an ESR spectrum at 100 K or lower of the oxide insulating film with asmall number of defects, a first signal that appears at a g-factor ofgreater than or equal to 2.037 and smaller than or equal to 2.039, asecond signal that appears at a g-factor of greater than or equal to2.001 and smaller than or equal to 2.003, and a third signal thatappears at a g-factor of greater than or equal to 1.964 and smaller thanor equal to 1.966 are observed. The split width between the first andsecond signals and the split width between the second and third signalsthat are obtained by ESR measurement using an X-band are eachapproximately 5 mT. The sum of the spin densities of the first signalthat appears at a g-factor of greater than or equal to 2.037 and smallerthan or equal to 2.039, the second signal that appears at a g-factor ofgreater than or equal to 2.001 and smaller than or equal to 2.003, andthe third signal that appears at a g-factor of greater than or equal to1.964 and smaller than or equal to 1.966 is lower than 1×10¹⁸ spins/cm³,typically higher than or equal to 1×10¹⁷ spins/cm³ and lower than 1×10¹⁸spins/cm³.

In the ESR spectrum at 100 K or lower, the first signal that appears ata g-factor of greater than or equal to 2.037 and smaller than or equalto 2.039, the second signal that appears at a g-factor of greater thanor equal to 2.001 and smaller than or equal to 2.003, and the thirdsignal that appears at a g-factor of greater than or equal to 1.964 andsmaller than or equal to 1.966 correspond to signals attributed tonitrogen oxide (NO_(x); x is greater than or equal to 0 and smaller thanor equal to 2, preferably greater than or equal to 1 and smaller than orequal to 2). Typical examples of nitrogen oxide include nitrogenmonoxide and nitrogen dioxide. In other words, when the spin densitiesof signals that appear at a g-factor of greater than or equal to 1.964and smaller than or equal to 1.966 to a g-factor of greater than orequal to 2.037 and smaller than or equal to 2.039 are lower, thenitrogen oxide content in an oxide insulating film is lower.

When the oxide insulating film 23 contains a small amount of nitrogenoxide as described above, the carrier trap at the interface between theoxide insulating film 23 and the oxide semiconductor film can bereduced. Thus, a change in the threshold voltage of the transistorincluded in the semiconductor device can be reduced, which leads to areduced change in the electrical characteristics of the transistor.

The oxide insulating film 23 preferably has a nitrogen concentrationmeasured by secondary ion mass spectrometry (SIMS) of lower than orequal to 6×10²⁰ atoms/cm³. In that case, nitrogen oxide is unlikely tobe generated in the oxide insulating film 23, so that the carrier trapat the interface between the oxide insulating film 23 and the oxidesemiconductor film 19 a can be reduced. Furthermore, a change in thethreshold voltage of the transistor included in the semiconductor devicecan be reduced, which leads to a reduced change in the electricalcharacteristics of the transistor.

Note that when nitride oxide and ammonia are contained in the oxideinsulating film 23, the nitride oxide and ammonia react with each otherin heat treatment in the manufacturing process; accordingly, the nitrideoxide is released as a nitrogen gas. Thus, the nitrogen concentration ofthe oxide insulating film 23 and the amount of nitrogen oxide thereincan be reduced. Moreover, the carrier trap at the interface between theoxide insulating film 23 and the oxide semiconductor film 19 a can bereduced. Furthermore, the amount of change in threshold voltage of thetransistor included in the semiconductor device can be reduced, whichleads to a reduced change in the electrical characteristics of thetransistor.

Note that in the oxide insulating film 23, all oxygen entering the oxideinsulating film 23 from the outside does not move to the outside of theoxide insulating film 23 and some oxygen remains in the oxide insulatingfilm 23. Furthermore, movement of oxygen occurs in the oxide insulatingfilm 23 in some cases in such a manner that oxygen enters the oxideinsulating film 23 and oxygen contained in the oxide insulating film 23is moved to the outside of the oxide insulating film 23.

When the oxide insulating film through which oxygen passes is formed asthe oxide insulating film 23, oxygen released from the oxide insulatingfilm 25 provided over the oxide insulating film 23 can be moved to theoxide semiconductor film 19 a through the oxide insulating film 23.

The oxide insulating film 25 is formed in contact with the oxideinsulating film 23. The oxide insulating film 25 is formed using anoxide insulating film which contains oxygen at a higher proportion thanthe stoichiometric composition. Part of oxygen is released by heatingfrom the oxide insulating film which contains oxygen at a higherproportion than the stoichiometric composition. The oxide insulatingfilm which contains oxygen at a higher proportion than thestoichiometric composition is an oxide insulating film of which theamount of released oxygen converted into oxygen atoms is greater than orequal to 1.0×10¹⁸ atoms/cm³, preferably greater than or equal to3.0×10²⁰ atoms/cm³ in TDS analysis. Note that the temperature of thefilm surface in the TDS analysis is preferably higher than or equal to100° C. and lower than or equal to 700° C., or higher than or equal to100° C. and lower than or equal to 500° C.

A silicon oxide film, a silicon oxynitride film, or the like with athickness greater than or equal to 30 nm and less than or equal to 500nm, preferably greater than or equal to 50 nm and less than or equal to400 nm can be used as the oxide insulating film 25.

It is preferable that the amount of defects in the oxide insulating film25 be small and typically, the spin density of a signal that appears atg=2.001 be lower than 1.5×10¹⁸ spins/cm³, further preferably lower thanor equal to 1×10¹⁸ spins/cm³ by ESR measurement. Note that the oxideinsulating film 25 is provided more apart from the oxide semiconductorfilm 19 a than the oxide insulating film 23 is; thus, the oxideinsulating film 25 may have higher defect density than the oxideinsulating film 23.

Like the nitride insulating film 15, the nitride insulating film 27 canbe a nitride insulating film which is hardly permeated by oxygen.Furthermore, a nitride insulating film which is hardly permeated byoxygen, hydrogen, and water can be used.

The nitride insulating film 27 is formed using a silicon nitride film, asilicon nitride oxide film, an aluminum nitride film, an aluminumnitride oxide film, or the like with a thickness greater than or equalto 50 nm and less than or equal to 300 nm, preferably greater than orequal to 100 nm and less than or equal to 200 nm.

In the case where the oxide insulating film which contains oxygen at ahigher proportion than the stoichiometric composition is included in theoxide insulating film 23 or the oxide insulating film 25, part of oxygencontained in the oxide insulating film 23 or the oxide insulating film25 can be moved to the oxide semiconductor film 19 a, so that the amountof oxygen vacancies contained in the oxide semiconductor film 19 a canbe reduced.

The threshold voltage of a transistor using an oxide semiconductor filmwith oxygen vacancies shifts negatively with ease, and such a transistortends to be normally on. This is because charges are generated owing tooxygen vacancies in the oxide semiconductor film and the resistance isthus reduced. The transistor having normally-on characteristic causesvarious problems in that malfunction is likely to be caused when inoperation and that power consumption is increased when not in operation,for example. Furthermore, there is a problem in that the amount ofchange in electrical characteristics, typically in threshold voltage, ofthe transistor is increased by change over time or due to a stress test.

However, in the transistor 102 in this embodiment, the oxide insulatingfilm 23 or the oxide insulating film 25 provided over the oxidesemiconductor film 19 a contains oxygen at a higher proportion than thestoichiometric composition. Furthermore, the oxide semiconductor film 19a, the oxide insulating film 23, and the oxide insulating film 25 aresurrounded by the nitride insulating film 15 and the oxide insulatingfilm 17. As a result, oxygen contained in the oxide insulating film 23or the oxide insulating film 25 is moved to the oxide semiconductor film19 a efficiently, so that the amount of oxygen vacancies in the oxidesemiconductor film 19 a can be reduced. Accordingly, a transistor havingnormally-off characteristic is obtained. Furthermore, the amount ofchange in electrical characteristics, typically in threshold voltage, ofthe transistor over time or due to a stress test can be reduced.

The common electrode 29 is formed using a light-transmitting film,preferably a light-transmitting conductive film. As thelight-transmitting conductive film, an indium oxide film containingtungsten oxide, an indium zinc oxide film containing tungsten oxide, anindium oxide film containing titanium oxide, an indium tin oxide filmcontaining titanium oxide, an indium tin oxide (hereinafter referred toas ITO) film, an indium zinc oxide film, an indium tin oxide film towhich silicon oxide is added, and the like are given.

The extending direction of the conductive film 21 a functioning as asignal line and the extending direction of the common electrode 29intersect with each other. Therefore, differences in directions betweenthe electric field between the conductive film 21 a functioning as asignal line and the common electrode 29 and the electric field betweenthe pixel electrode 19 b and the common electrode 29 arise and thedifferences form a large angle. Accordingly, in the case where negativeliquid crystal molecules are used, the alignment state of the liquidcrystal molecules in the vicinity of the conductive film functioning asa signal line and the alignment state of the liquid crystal molecules inthe vicinity of the pixel electrode which is generated by an electricfield between the pixel electrodes provided in adjacent pixels and thecommon electrode are less likely to be affected by each other. Thus, achange in the transmittance of the pixels is suppressed. Accordingly,flickers in an image can be reduced.

In the liquid crystal display device having a low refresh rate,alignment of liquid crystal molecules in the vicinity of the conductivefilm 21 a functioning as a signal line is less likely to affectalignment state of liquid crystal molecules in the vicinity of the pixelelectrode due to the electric field between the pixel electrodes in theadjacent pixels and the common electrode 29 even during the retentionperiod. Thus, the transmittance of the pixels in the retention periodcan be held and flickers can be reduced.

The common electrode 29 includes the stripe regions extending in adirection intersecting with the conductive film 21 a functioning as asignal line. Accordingly, in the vicinity of the pixel electrode 19 band the conductive film 21 a, unintended alignment of liquid crystalmolecules can be prevented and thus light leakage can be suppressed. Asa result, a display device with excellent contrast can be manufactured.

Note that the shape of the common electrode 29 is not limited to thatillustrated in FIG. 4, and may be stripe. In the case of a stripe shape,the extending direction may be parallel to the conductive filmfunctioning as a signal line. The common electrode 29 may have a combshape. Alternatively, the common electrode may be formed over the entiresurface of the first substrate 11. Further alternatively, alight-transmitting conductive film different from the pixel electrode 19b may be formed over the common electrode 29 with an insulating filmprovided therebetween.

The alignment film 33 is formed over the common electrode 29, thenitride insulating film 27, and the organic insulating film 31.

Next, a method for manufacturing the transistor 102 and the capacitor105 in FIG. 5 is described with reference to FIGS. 10A to 10D, FIGS. 11Ato 11D, and FIGS. 12A to 12C.

As illustrated in FIG. 10A, a conductive film 12 to be the conductivefilm 13 is formed over the first substrate 11. The conductive film 12 isformed by a sputtering method, a chemical vapor deposition (CVD) methodsuch as a metal organic chemical vapor deposition (MOCVD) method, ametal chemical deposition method, an atomic layer deposition (ALD)method, or a plasma-enhanced chemical vapor deposition (PECVD) method,an evaporation method, a pulsed laser deposition (PLD) method, or thelike. When a metal organic chemical vapor deposition (MOCVD) method, ametal chemical deposition method, or an atomic layer deposition (ALD)method is employed, the conductive film is less damaged by plasma.

Here, a glass substrate is used as the first substrate 11. Furthermore,as the conductive film 12, a 100-nm-thick tungsten film is formed by asputtering method.

Next, a mask is formed over the conductive film 12 by a photolithographyprocess using a first photomask. Then, as illustrated in FIG. 10B, partof the conductive film 12 is etched with the use of the mask to form theconductive film 13 functioning as a gate electrode. After that, the maskis removed.

Note that the conductive film 13 functioning as a gate electrode may beformed by an electrolytic plating method, a printing method, an ink jetmethod, or the like instead of the above formation method.

Here, the tungsten film is etched by a dry etching method to form theconductive film 13 functioning as a gate electrode.

Next, as illustrated in FIG. 10C, over the conductive film 13functioning as a gate electrode, the nitride insulating film 15 and anoxide insulating film 16 to be the oxide insulating film 17 later areformed. Then, over the oxide insulating film 16, an oxide semiconductorfilm 18 to be the oxide semiconductor film 19 a and the pixel electrode19 b later is formed.

The nitride insulating film 15 and the oxide insulating film 16 are eachformed by a sputtering method, a chemical vapor deposition (CVD) methodsuch as a metal organic chemical vapor deposition (MOCVD) method, ametal chemical deposition method, an atomic layer deposition (ALD)method, or a plasma-enhanced chemical vapor deposition (PECVD) method,an evaporation method, a pulsed laser deposition (PLD) method, a coatingmethod, a printing method, or the like. When a metal organic chemicalvapor deposition (MOCVD) method, a metal chemical deposition method, oran atomic layer deposition (ALD) method is employed, the nitrideinsulating film 15 and the oxide insulating film 16 are less damaged byplasma. When an atomic layer deposition (ALD) method is employed,coverage of the nitride insulating film 15 and the oxide insulating film16 can be increased.

Here, as the nitride insulating film 15, a 300-nm-thick silicon nitridefilm is formed by a plasma CVD method in which silane, nitrogen, andammonia are used as a source gas.

In the case where a silicon oxide film, a silicon oxynitride film, or asilicon nitride oxide film is formed as the oxide insulating film 16, adeposition gas containing silicon and an oxidizing gas are preferablyused as a source gas. Typical examples of the deposition gas containingsilicon include silane, disilane, trisilane, and silane fluoride. As theoxidizing gas, oxygen, ozone, dinitrogen monoxide, and nitrogen dioxidecan be given as examples.

Moreover, in the case of forming a gallium oxide film as the oxideinsulating film 16, a metal organic chemical vapor deposition (MOCVD)method can be employed.

Here, as the oxide insulating film 16, a 50-nm-thick silicon oxynitridefilm is formed by a plasma CVD method in which silane and dinitrogenmonoxide are used as a source gas.

The oxide semiconductor film 18 can be formed by a sputtering method, achemical vapor deposition (CVD) method such as a metal organic chemicalvapor deposition (MOCVD) method, an atomic layer deposition (ALD)method, or a plasma-enhanced chemical vapor deposition (PECVD) method, apulsed laser deposition method, a laser ablation method, a coatingmethod, or the like. When a metal organic chemical vapor deposition(MOCVD) method, a metal chemical deposition method, or an atomic layerdeposition (ALD) method is employed, the oxide semiconductor film 18 isless damaged by plasma and the oxide insulating film 16 is less damaged.When an atomic layer deposition (ALD) method is employed, coverage ofthe oxide semiconductor film 18 can be increased.

As a power supply device for generating plasma in the case of formingthe oxide semiconductor film by a sputtering method, an RF power supplydevice, an AC power supply device, a DC power supply device, or the likecan be used as appropriate.

As a sputtering gas, a rare gas (typically argon), an oxygen gas, or amixed gas of a rare gas and oxygen is used as appropriate. In the caseof using the mixed gas of a rare gas and oxygen, the proportion ofoxygen to a rare gas is preferably increased.

Furthermore, a target may be selected as appropriate in accordance withthe composition of the oxide semiconductor film to be formed.

In order to obtain a highly purified intrinsic or substantially highlypurified intrinsic oxide semiconductor film, it is necessary to highlypurify a sputtering gas as well as to evacuate a chamber to a highvacuum. As an oxygen gas or an argon gas used for a sputtering gas, agas which is highly purified to have a dew point of −40° C. or lower,preferably −80° C. or lower, further preferably −100° C. or lower, stillfurther preferably −120° C. or lower is used, whereby entry of moistureor the like into the oxide semiconductor film can be prevented as muchas possible.

Here, a 35-nm-thick In—Ga—Zn oxide film is formed as the oxidesemiconductor film by a sputtering method using an In—Ga—Zn oxide target(In:Ga:Zn=1:1:1).

Then, after a mask is formed over the oxide semiconductor film 18 by aphotolithography process using a second photomask, the oxidesemiconductor film is partly etched using the mask. Thus, the oxidesemiconductor film 19 a and an oxide semiconductor film 19 c which areisolated from each other as illustrated in FIG. 10D are formed. Afterthat, the mask is removed.

Here, the oxide semiconductor films 19 a and 19 c are formed in such amanner that a mask is formed over the oxide semiconductor film 18 andpart of the oxide semiconductor film 18 is selectively etched by a wetetching method.

Next, as illustrated in FIG. 11A, a conductive film 20 to be theconductive films 21 a and 21 b later is formed.

The conductive film 20 can be formed by a method similar to that of theconductive film 12 as appropriate.

Here, a 50-nm-thick tungsten film and a 300-nm-thick copper film aresequentially stacked by a sputtering method.

Next, a mask is formed over the conductive film 20 by a photolithographyprocess using a third photomask. Then, the conductive film 20 is etchedusing the mask, so that the conductive films 21 a and 21 b functioningas a source electrode and a drain electrode are formed as illustrated inFIG. 11B. After that, the mask is removed.

Here, the mask is formed over the copper film by a photolithographyprocess. Then, the tungsten film and the copper film are etched with theuse of the mask, so that the conductive films 21 a and 21 b are formed.Note that the copper film is etched by a wet etching method. Next, thetungsten film is etched by a dry etching method using SF₆, wherebyfluoride is formed on the surface of the copper film. By the fluoride,diffusion of copper elements from the copper film is reduced and thusthe copper concentration in the oxide semiconductor film 19 a can bereduced.

Next, as illustrated in FIG. 11C, an oxide insulating film 22 to be theoxide insulating film 23 later and an oxide insulating film 24 to be theoxide insulating film 25 later are formed over the oxide semiconductorfilms 19 a and 19 c and the conductive films 21 a and 21 b. The oxideinsulating film 22 and the oxide insulating film 24 can each be formedby a method similar to those of the nitride insulating film 15 and theoxide insulating film 16 as appropriate.

Note that after the oxide insulating film 22 is formed, the oxideinsulating film 24 is preferably formed in succession without exposureto the air. After the oxide insulating film 22 is formed, the oxideinsulating film 24 is formed in succession by adjusting at least one ofthe flow rate of a source gas, pressure, a high-frequency power, and asubstrate temperature without exposure to the air, whereby theconcentration of impurities attributed to the atmospheric component atthe interface between the oxide insulating film 22 and the oxideinsulating film 24 can be reduced and oxygen in the oxide insulatingfilm 24 can be moved to the oxide semiconductor film 19 a; accordingly,the amount of oxygen vacancies in the oxide semiconductor film 19 a canbe reduced.

The oxide insulating film 22 can be formed using an oxide insulatingfilm containing nitrogen and having a small number of defects which isformed by a CVD method under the conditions where the ratio of anoxidizing gas to a deposition gas is higher than 20 times and lower than100 times, preferably higher than or equal to 40 times and lower than orequal to 80 times and the pressure in a treatment chamber is lower than100 Pa, preferably lower than or equal to 50 Pa.

A deposition gas containing silicon and an oxidizing gas are preferablyused as the source gas of the oxide insulating film 22. Typical examplesof the deposition gas containing silicon include silane, disilane,trisilane, and silane fluoride. As the oxidizing gas, oxygen, ozone,dinitrogen monoxide, and nitrogen dioxide can be given as examples.

With the use of the above conditions, an oxide insulating film whichpermeates oxygen can be formed as the oxide insulating film 22.Furthermore, by providing the oxide insulating film 22, damage to theoxide semiconductor film 19 a can be reduced in the step of forming theoxide insulating film 24.

Here, as the oxide insulating film 22, a 50-nm-thick silicon oxynitridefilm is formed by a plasma CVD method in which silane with a flow rateof 50 sccm and dinitrogen monoxide with a flow rate of 2000 sccm areused as a source gas, the pressure in the treatment chamber is 20 Pa,the substrate temperature is 220° C., and a high-frequency power of 100W is supplied to parallel-plate electrodes with the use of a 27.12 MHzhigh-frequency power source. Under the above conditions, a siliconoxynitride film containing nitrogen and having a small number of defectscan be formed.

As the oxide insulating film 24, a silicon oxide film or a siliconoxynitride film is formed under the following conditions: the substrateplaced in a treatment chamber of a plasma CVD apparatus that isvacuum-evacuated is held at a temperature higher than or equal to 180°C. and lower than or equal to 280° C., preferably higher than or equalto 200° C. and lower than or equal to 240° C., the pressure is greaterthan or equal to 100 Pa and less than or equal to 250 Pa, preferablygreater than or equal to 100 Pa and less than or equal to 200 Pa withintroduction of a source gas into the treatment chamber, and ahigh-frequency power of greater than or equal to 0.17 W/cm² and lessthan or equal to 0.5 W/cm², preferably greater than or equal to 0.25W/cm² and less than or equal to 0.35 W/cm² is supplied to an electrodeprovided in the treatment chamber.

A deposition gas containing silicon and an oxidizing gas are preferablyused as the source gas of the oxide insulating film 24. Typical examplesof the deposition gas containing silicon include silane, disilane,trisilane, and silane fluoride. As the oxidizing gas, oxygen, ozone,dinitrogen monoxide, and nitrogen dioxide can be given as examples.

As the film formation conditions of the oxide insulating film 24, thehigh-frequency power having the above power density is supplied to thetreatment chamber having the above pressure, whereby the degradationefficiency of the source gas in plasma is increased, oxygen radicals areincreased, and oxidation of the source gas is promoted; therefore, theoxygen content in the oxide insulating film 24 becomes higher than thatin the stoichiometric composition. On the other hand, in the film formedat a substrate temperature within the above temperature range, the bondbetween silicon and oxygen is weak, and accordingly, part of oxygen inthe film is released by heat treatment in a later step. Thus, it ispossible to form an oxide insulating film which contains oxygen at ahigher proportion than the stoichiometric composition and from whichpart of oxygen is released by heating. Furthermore, the oxide insulatingfilm 22 is provided over the oxide semiconductor film 19 a. Accordingly,in the step of forming the oxide insulating film 24, the oxideinsulating film 22 serves as a protective film of the oxidesemiconductor film 19 a. Consequently, the oxide insulating film 24 canbe formed using the high-frequency power having a high power densitywhile damage to the oxide semiconductor film 19 a is reduced.

Here, as the oxide insulating film 24, a 400-nm-thick silicon oxynitridefilm is formed by a plasma CVD method in which silane with a flow rateof 200 sccm and dinitrogen monoxide with a flow rate of 4000 sccm areused as the source gas, the pressure in the treatment chamber is 200 Pa,the substrate temperature is 220° C., and a high-frequency power of 1500W is supplied to the parallel-plate electrodes with the use of a 27.12MHz high-frequency power source. Note that the plasma CVD apparatus is aparallel-plate plasma CVD apparatus in which the electrode area is 6000cm², and the power per unit area (power density) into which the suppliedpower is converted is 0.25 W/cm².

Furthermore, when the conductive films 21 a and 21 b functioning as asource electrode and a drain electrode is formed, the oxidesemiconductor film 19 a is damaged by the etching of the conductivefilm, so that oxygen vacancies are generated on the back channel side ofthe oxide semiconductor film 19 a (the side of the oxide semiconductorfilm 19 a which is opposite to the side facing the conductive film 13functioning as a gate electrode). However, with the use of the oxideinsulating film which contains oxygen at a higher proportion than thestoichiometric composition as the oxide insulating film 24, the oxygenvacancies generated on the back channel side can be repaired by heattreatment. By this, defects contained in the oxide semiconductor film 19a can be reduced, and thus, the reliability of the transistor 102 can beimproved.

Then, a mask is formed over the oxide insulating film 24 by aphotolithography process using a fourth photomask. Next, as illustratedin FIG. 11D, part of the oxide insulating film 22 and part of the oxideinsulating film 24 are etched with the use of the mask to form the oxideinsulating film 23 and the oxide insulating film 25 having the opening40. After that, the mask is removed.

In the process, the oxide insulating films 22 and 24 are preferablyetched by a dry etching method. As a result, the oxide semiconductorfilm 19 c is exposed to plasma in the etching treatment; thus, theamount of oxygen vacancies in the oxide semiconductor film 19 c can beincreased.

Next, heat treatment is performed. The heat treatment is performedtypically at a temperature higher than or equal to 150° C. and lowerthan or equal to 400° C., preferably higher than or equal to 300° C. andlower than or equal to 400° C., further preferably higher than or equalto 320° C. and lower than or equal to 370° C.

An electric furnace, an RTA apparatus, or the like can be used for theheat treatment. With the use of an RTA apparatus, the heat treatment canbe performed at a temperature higher than or equal to the strain pointof the substrate if the heating time is short. Therefore, the heattreatment time can be shortened.

The heat treatment may be performed under an atmosphere of nitrogen,oxygen, ultra-dry air (air in which a water content is 20 ppm or less,preferably 1 ppm or less, further preferably 10 ppb or less), or a raregas (argon, helium, or the like). The atmosphere of nitrogen, oxygen,ultra-dry air, or a rare gas preferably does not contain hydrogen,water, and the like.

By the heat treatment, part of oxygen contained in the oxide insulatingfilm 25 can be moved to the oxide semiconductor film 19 a, so that theamount of oxygen vacancies contained in the oxide semiconductor film 19a can be further reduced.

In the case where water, hydrogen, or the like enters the oxideinsulating film 23 and the oxide insulating film 25 and the nitrideinsulating film 26 has a barrier property against water, hydrogen, orthe like, when the nitride insulating film 26 is formed later and heattreatment is performed, water, hydrogen, or the like contained in theoxide insulating film 23 and the oxide insulating film 25 are moved tothe oxide semiconductor film 19 a, so that defects are generated in theoxide semiconductor film 19 a. However, by the heating, water, hydrogen,or the like contained in the oxide insulating film 23 and the oxideinsulating film 25 can be released; thus, variation in electricalcharacteristics of the transistor 102 can be reduced, and a change inthreshold voltage can be suppressed.

Note that when the oxide insulating film 24 is formed over the oxideinsulating film 22 while being heated, oxygen can be moved to the oxidesemiconductor film 19 a to reduce the amount of oxygen vacancies in theoxide semiconductor film 19 a; thus, the heat treatment is notnecessarily performed.

The heat treatment may be performed after the formation of the oxideinsulating films 22 and 24. However, the heat treatment is preferablyperformed after the formation of the oxide insulating films 23 and 25because a film having higher conductivity can be formed in such a mannerthat oxygen is not moved to the oxide semiconductor film 19 c and oxygenis released from the oxide semiconductor film 19 c because of exposureof the oxide semiconductor film 19 c and then oxygen vacancies aregenerated.

Here, the heat treatment is performed at 350° C. in a mixed atmosphereof nitrogen and oxygen for one hour.

Then, as illustrated in FIG. 12A, the nitride insulating film 26 isformed.

The nitride insulating film 26 can be formed by a method similar tothose of the nitride insulating film 15 and the oxide insulating film 16as appropriate. By forming the nitride insulating film 26 by asputtering method, a CVD method, or the like, the oxide semiconductorfilm 19 c is exposed to plasma; thus, the amount of oxygen vacancies inthe oxide semiconductor film 19 c can be increased.

The oxide semiconductor film 19 c has improved conductivity andfunctions as the pixel electrode 19 b. When a silicon nitride film isformed by a plasma CVD method as the nitride insulating film 26,hydrogen contained in the silicon nitride film is diffused to the oxidesemiconductor film 19 c; thus, the conductivity of the pixel electrode19 b can be enhanced.

In the case where a silicon nitride film is formed by a plasma CVDmethod as the nitride insulating film 26, the substrate placed in thetreatment chamber of the plasma CVD apparatus that is vacuum-evacuatedis preferably held at a temperature higher than or equal to 300° C. andlower than or equal to 400° C., further preferably higher than or equalto 320° C. and lower than or equal to 370° C., so that a dense siliconnitride film can be formed.

In the case where a silicon nitride film is formed, a deposition gascontaining silicon, nitrogen, and ammonia are preferably used as asource gas. As the source gas, a small amount of ammonia compared to theamount of nitrogen is used, whereby ammonia is dissociated in the plasmaand activated species are generated. The activated species cleave a bondbetween silicon and hydrogen which are contained in a deposition gascontaining silicon and a triple bond between nitrogen molecules. As aresult, a dense silicon nitride film having few defects, in which bondsbetween silicon and nitrogen are promoted and bonds between silicon andhydrogen is few, can be formed. On the other hand, when the amount ofammonia is larger than the amount of nitrogen in the source gas,cleavage of a deposition gas containing silicon and cleavage of nitrogenare not promoted, so that a sparse silicon nitride film in which bondsbetween silicon and hydrogen remain and defects are increased is formed.Therefore, in a source gas, the flow ratio of the nitrogen to theammonia is set to be preferably greater than or equal to 5 and less thanor equal to 50, further preferably greater than or equal to 10 and lessthan or equal to 50.

Here, in the treatment chamber of a plasma CVD apparatus, a 50-nm-thicksilicon nitride film is formed as the nitride insulating film 26 by aplasma CVD method in which silane with a flow rate of 50 sccm, nitrogenwith a flow rate of 5000 sccm, and ammonia with a flow rate of 100 sccmare used as the source gas, the pressure in the treatment chamber is 100Pa, the substrate temperature is 350° C., and a high-frequency power of1000 W is supplied to parallel-plate electrodes with a high-frequencypower supply of 27.12 MHz. Note that the plasma CVD apparatus is aparallel-plate plasma CVD apparatus in which the electrode area is 6000cm², and the power per unit area (power density) into which the suppliedpower is converted is 1.7×10⁻¹ W/cm².

Next, heat treatment may be performed. The heat treatment is performedtypically at a temperature higher than or equal to 150° C. and lowerthan or equal to 400° C., preferably higher than or equal to 300° C. andlower than or equal to 400° C., further preferably higher than or equalto 320° C. and lower than or equal to 370° C. As a result, the negativeshift of the threshold voltage can be reduced. Moreover, the amount ofchange in the threshold voltage can be reduced.

Next, although not illustrated, a mask is formed by a photolithographyprocess using a fifth photomask. Then, part of each of the nitrideinsulating film 15, the oxide insulating film 16, the oxide insulatingfilm 23, the oxide insulating film 25, and the nitride insulating film26 is etched using the mask to form the nitride insulating film 27 andan opening through which part of a connection terminal formed at thesame time as the conductive film 13 is exposed. Alternatively, part ofeach of the oxide insulating film 23, the oxide insulating film 25, andthe nitride insulating film 26 is etched to form the nitride insulatingfilm 27 and an opening through which part of a connection terminalformed at the same time as the conductive films 21 a and 21 b isexposed.

Next, as illustrated in FIG. 12B, a conductive film 28 to be the commonelectrode 29 later is formed over the nitride insulating film 27.

The conductive film 28 is formed by a sputtering method, a CVD method,an evaporation method, or the like.

Then, a mask is formed over the conductive film 28 by a photolithographyprocess using a sixth photomask. Next, as illustrated in FIG. 12C, partof the conductive film 28 is etched with the use of the mask to form thecommon electrode 29. Although not illustrated, the common electrode 29is connected to the connection terminal formed at the same time as theconductive film 13 or the connection terminal formed at the same time asthe conductive films 21 a and 21 b. After that, the mask is removed.

Next, as illustrated in FIG. 13, the organic insulating film 31 isformed over the nitride insulating film 27. An organic insulating filmcan be formed by a coating method, a printing method, or the like asappropriate.

In the case where the organic insulating film is formed by a coatingmethod, a photosensitive composition, with which the upper surfaces ofthe nitride insulating film 27 and the common electrode 29 are coated,is exposed to light and developed by photolithography process using aseventh photomask, and is then subjected to heat treatment. Note that inthe case where the upper surfaces of the nitride insulating film 27 andthe common electrode 29 are coated with a non-photosensitivecomposition, a resist, with which the upper surface of thenon-photosensitive composition is coated, is processed by aphotolithography process using a seventh mask to form a mask, and thenthe non-photosensitive composition is etched using the mask, whereby theorganic insulating film 31 can be formed.

Through the above process, the transistor 102 is manufactured and thecapacitor 105 can be manufactured.

The element substrate of the display device described in this embodimentincludes an organic insulating film overlapping with a transistor withan inorganic insulating film provided therebetween. Therefore, a displaydevice in which reliability of the transistor can be improved and whosedisplay quality is maintained can be manufactured.

The element substrate of the display device of this embodiment isprovided with a common electrode whose upper surface has a zigzag shapeand which includes stripe regions extending in a direction intersectingwith the conductive film functioning as a signal line. Therefore, thedisplay device can have excellent contrast. In addition, flickers can bereduced in a liquid crystal display device having a low refresh rate.

In the element substrate of the display device of this embodiment, thepixel electrode is formed at the same time as the oxide semiconductorfilm of the transistor; therefore, the transistor 102 and the capacitor105 can be formed using six photomasks. The pixel electrode functions asthe one of electrodes of the capacitor. The common electrode alsofunctions as the other of electrodes of the capacitor. Thus, a step offorming another conductive film is not needed to form the capacitor, andthe number of steps of manufacturing the display device can be reduced.The capacitor has a light-transmitting property. As a result, the areaoccupied by the capacitor can be increased and the aperture ratio in apixel can be increased. Moreover, power consumption of the displaydevice can be reduced.

Next, the element layer formed on the second substrate 342 is described.A film having a colored property (hereinafter referred to as a coloringfilm 346) is formed on the second substrate 342. The coloring film 346functions as a color filter. Furthermore, a light-blocking film 344adjacent to the coloring film 346 is formed on the second substrate 342.The light-blocking film 344 functions as a black matrix. The coloringfilm 346 is not necessarily provided in the case where the liquidcrystal display device is a monochrome display device, for example.

The coloring film 346 is a coloring film that transmits light in aspecific wavelength range. For example, a red (R) film for transmittinglight in a red wavelength range, a green (G) film for transmitting lightin a green wavelength range, a blue (B) film for transmitting light in ablue wavelength range, or the like can be used.

The light-blocking film 344 preferably has a function of blocking lightin a specific wavelength range, and can be a metal film or an organicinsulating film including a black pigment or the like, for example.

An insulating film 348 is formed on the coloring film 346. Theinsulating film 348 functions as a planarization layer or suppressesdiffusion of impurities in the coloring film 346 to the liquid crystalelement side.

A conductive film 350 is formed on the insulating film 348. Theconductive film 350 is formed using a light-transmitting conductivefilm. The potential of the conductive film 350 is preferably the same asthat of the common electrode 29. In other words, a common potential ispreferably applied to the conductive film 350.

When a voltage for driving the liquid crystal molecules is applied tothe conductive film 21 b, an electric field is generated between theconductive film 21 b and the common electrode 29. Liquid crystalmolecules between the conductive film 21 b and the common electrode 29align due to the effect of the electric field and thus a flicker isgenerated.

However, it is possible to suppress an alignment change of liquidcrystal molecules in a direction perpendicular to the substrate due toan electric field between the conductive film 21 b and the commonelectrode 29 in such a manner that the conductive film 350 is providedto face the common electrode 29 through the liquid crystal layer 320 sothat the common electrode 29 and the conductive film 350 have the samepotential. Accordingly, the alignment state of the liquid crystalmolecules in the region is stabilized. Thus, flickers can be reduced.

Note that the alignment film 352 is formed on the conductive film 350.

In addition, the liquid crystal layer 320 is formed between thealignment films 33 and 352. The liquid crystal layer 320 is sealedbetween the first substrate 11 and the second substrate 342 with the useof a sealant (not illustrated). The sealant is preferably in contactwith an inorganic material to prevent entry of moisture and the likefrom the outside.

A spacer may be provided between the alignment films 33 and 352 tomaintain the thickness of the liquid crystal layer 320 (also referred toas a cell gap).

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments.

Modification Example 1

A structure in which a common line connected to the common electrode isprovided in the display device described in Embodiment 1 is describedwith reference to FIGS. 14A and 14B.

FIG. 14A is a top view of the pixels 103 a, 103 b, and 103 c included ina display device, and FIG. 14B is a cross-sectional view taken alongdashed-dotted lines A-B and C-D in FIG. 14A.

As illustrated in FIG. 14A, the upper surface of the common electrode 29has a zigzag shape, and the extending direction of the conductive film21 a functioning as a signal line intersects with the extendingdirection of the common electrode 29.

For easy understanding of the structure of the common electrode 29, thecommon electrode 29 is hatched in FIG. 14A to explain its shape. Thecommon electrode 29 includes regions hatched diagonally left down and aregion hatched diagonally right down. The regions hatched diagonallyleft down are stripe regions (first regions) having a zigzag shape, andthe extending direction of the conductive film 21 a functioning as asignal line intersects with the extending direction of the commonelectrode 29. The region hatched diagonally right down is a connectionregion (second region) connected to the stripe regions (first regions)and extending in a direction parallel or substantially parallel to theconductive film 21 a functioning as a signal line.

A common line 21 c overlaps with the connection region (second region)of the common electrode 29.

Alternatively, the common line 21 c may be provided every plurality ofpixels. Alternatively, the common line 21 c may be provided everyplurality of pixels. For example, as illustrated in FIG. 14A, one commonline 21 c is provided for every three pixels, so that the area occupiedby the common line in the flat plane of the display device can bereduced. As a result, the area of the pixel and the aperture ratio ofthe pixel can be increased.

In a region where the pixel electrode 19 b and the common electrode 29overlap with each other, a liquid crystal molecule is less likely to bedriven by an electric field generated between the pixel electrode 19 band the connection region (second region) of the common electrode 29.Therefore, the area of a region overlapping with the pixel electrode 19b in the connection region (second region) of the common electrode 29 isreduced, so that a region where a liquid crystal molecule is driven canbe increased, leading to an increase in the aperture ratio. For example,as illustrated in FIG. 14A, the connection region (second region) of thecommon electrode 29 is provided so as not to overlap with the pixelelectrode 19 b, whereby the area of a region where the pixel electrode19 b and the common electrode 29 overlap with each other can be reducedand thus the aperture ratio of the pixel can be increased.

Although one common line 21 c is provided for the three pixels 103 a,103 b, and 103 c in FIG. 14A, one common line 21 c may be provided forevery two pixels. Alternatively, one common line 21 c may be providedfor every four or more pixels.

As illustrated in FIG. 14B, the common line 21 c can be formed at thesame time as the conductive film 21 a functioning as a signal line. Thecommon electrode 29 is connected to the common line 21 c in an opening42 formed in the oxide insulating film 23, the oxide insulating film 25,and the nitride insulating film 27.

Since a material of the conductive film 21 a has resistivity lower thanthat of the common electrode 29, resistance of the common electrode 29and the common line 21 c can be reduced.

Modification Example 2

This modification example is different from Embodiment 2 in that atransistor included in a high resolution display device includes asource electrode and a drain electrode capable of reducing lightleakage.

FIG. 15 is a top view of the display device described in thisembodiment. One of features of the display device is that the conductivefilm 21 b functioning as one of a source electrode and a drain electrodehas an L shape in the top view. In other words, the conductive film 21 bhas a shape in which a region 21 b_1 extending in a directionperpendicular or substantially perpendicular to the extending directionof the conductive film 13 functioning as a scan line and a region 21 b_2extending in a direction parallel or substantially parallel to theextending direction of the conductive film 13 are connected to eachother in the top view. The region 21 b_2 overlaps with at least one ofthe conductive film 13, the pixel electrode 19 b, and the commonelectrode 29 in the top view. Alternatively, the conductive film 21 bincludes the region 21 b_2 extending in a direction parallel orsubstantially parallel to the extending direction of the conductive film13 and the region 21 b_2 is placed between the conductive film 13 andthe pixel electrode 19 b or the common electrode 29 in the top view.

Since the area of the pixel in a high resolution display device isreduced, the distance between the common electrode 29 and the conductivefilm 13 functioning as a scan line is reduced. In a pixel performingblack display, when voltage at which a transistor is turned on isapplied to the conductive film 13 functioning as a scan line, anelectric field is generated between the pixel electrode 19 b in a blackdisplay state and the conductive film 13 functioning as a scan line. Asa result, a liquid crystal molecule rotates in an unintended direction,causing light leakage.

However, in the transistor included in the display device of thisembodiment, the conductive film 21 b functioning as one of a sourceelectrode and a drain electrode includes the region 21 b_2 overlappingwith at least one of the conductive film 13, the pixel electrode 19 b,and the common electrode 29, or the region 21 b_2 placed between theconductive film 13 and the pixel electrode 19 b or the common electrode29 in the top view. As a result, the region 21 b_2 blocks the electricfield of the conductive film 13 functioning as a scan line and anelectric field generated between the conductive film 13 and the pixelelectrode 19 b can be suppressed, leading to a reduction in lightleakage.

Note that the conductive film 21 b and the common electrode 29 mayoverlap with each other. The overlapping region can function as acapacitor. Therefore, with this structure, the capacitance of thecapacitor can be increased. FIG. 16 illustrates an example of this case.

Modification Example 3

This modification example is different from Embodiment 2 in that a highresolution display device includes a common electrode capable ofreducing light leakage.

FIG. 17 is a top view of the display device described in thisembodiment. A common electrode 29 a includes a stripe region 29 a_1extending in a direction intersecting with the conductive film 21 afunctioning as a signal line and a region 29 a_2 which is connected tothe stripe region and overlaps with the conductive film 13 functioningas a scan line.

Since the area of a pixel is reduced in a high resolution displaydevice, the distance between the pixel electrode 19 b and the conductivefilm 13 functioning as a scan line is reduced. When voltage is appliedto the conductive film 13 functioning as a scan line, an electric fieldis generated between the conductive film 13 and the pixel electrode 19b. As a result, a liquid crystal molecule rotates in an unintendeddirection, causing light leakage.

However, the display device described in this embodiment includes thecommon electrode 29 a including the region 29 a_2 intersecting with theconductive film 13 functioning as a scan line. Therefore, an electricfield is generated between the conductive film 13 and the commonelectrode 29 a, a liquid crystal molecule rotates due to an electricfield being generated between the pixel electrode 19 b and theconductive film 13 functioning as a scan line can be suppressed, leadingto a reduction in light leakage.

Note that the conductive film 21 b and the common electrode 29 a mayoverlap with each other. The overlapping region can function as acapacitor. Therefore, with this structure, the capacitance of thecapacitor can be increased. FIG. 18 illustrates an example of this case.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments.

Embodiment 3

In this embodiment, as an example of a display device, a liquid crystaldisplay device driven in a vertical alignment (VA) mode will bedescribed. First, a top view of a plurality of pixels 103 included inthe liquid crystal display device is shown in FIG. 19.

In FIG. 19, a conductive film 13 functioning as a scan line extends in adirection substantially perpendicularly to a conductive film functioningas a signal line (in the lateral direction in the drawing). Theconductive film 21 a functioning as a signal line extends in a directionsubstantially perpendicularly to the conductive film functioning as ascan line (in the longitudinal direction in the drawing). A conductivefilm 21 e functioning as a capacitor line extends in a directionparallel to the signal line. Note that the conductive film 13functioning as a scan line is electrically connected to the scan linedriver circuit 104 (see FIGS. 1A to 1E), and the conductive film 21 afunctioning as a signal line and the conductive film 21 e functioning asa capacitor line is electrically connected to the signal line drivercircuit 106 (see FIGS. 1A to 1E).

The transistor 102 is provided in a region where the conductive filmfunctioning as a scan line and the conductive film functioning as asignal line intersect with each other. The transistor 102 includes theconductive film 13 functioning as a gate electrode; a gate insulatingfilm (not illustrated in FIG. 19); the oxide semiconductor film 19 awhere a channel region is formed, over the gate insulating film; and theconductive films 21 a and 21 b functioning as a pair of electrodes. Theconductive film 13 also functions as a scan line, and a region of theconductive film 13 that overlaps with the oxide semiconductor film 19 afunctions as the gate electrode of the transistor 102. In addition, theconductive film 21 a also functions as a signal line, and a region ofthe conductive film 21 a that overlaps with the oxide semiconductor film19 a functions as the source electrode or the drain electrode of thetransistor 102. Furthermore, in the top view of FIG. 19, an end portionof the conductive film functioning as a scan line is positioned on anouter side of an end portion of the oxide semiconductor film 19 a. Thus,the conductive film functioning as a scan line functions as alight-blocking film for blocking light from a light source such as abacklight. For this reason, the oxide semiconductor film 19 a includedin the transistor is not irradiated with light, so that a variation inthe electrical characteristics of the transistor can be suppressed.

In addition, the transistor 102 includes the organic insulating film 31overlapping with the oxide semiconductor film 19 a in a manner similarto that in Embodiment 1. The organic insulating film 31 overlaps withthe oxide semiconductor film 19 a (in particular, a region of the oxidesemiconductor film 19 a which is between the conductive films 21 a and21 b) with an inorganic insulating film (not illustrated in FIG. 19)provided therebetween.

The conductive film 21 b is electrically connected to alight-transmitting conductive film 29 c that functions as a pixelelectrode in an opening 41.

The capacitor 105 is connected to the conductive film 21 e functioningas a capacitor line. The capacitor 105 includes a film 19 d havingconductivity formed over the gate insulating film, a dielectric filmformed over the transistor 102, and the light-transmitting conductivefilm 29 c functioning as a pixel electrode. The dielectric film isformed using an oxygen barrier film. The film 19 d having conductivityformed over the gate insulating film has a light-transmitting property.That is, the capacitor 105 has a light-transmitting property.

Owing to the light-transmitting property of the capacitor 105, thecapacitor 105 can be formed large (in a large area) in the pixel 103.Thus, a display device with a large-capacitance capacitor as well as anaperture ratio increased to typically 55% or more, preferably 60% ormore can be provided. For example, in a high-resolution display devicesuch as a liquid crystal display device, the area of a pixel is smalland accordingly the area of a capacitor is also small. For this reason,the amount of charges accumulated in the capacitor is small in thehigh-resolution display device. However, since the capacitor 105 of thisembodiment has a light-transmitting property, when the capacitor 105 isprovided in a pixel, sufficient capacitance can be obtained in the pixeland the aperture ratio can be improved. Typically, the capacitor 105 canbe favorably used for a high-resolution display device with a pixeldensity of 200 pixels per inch (ppi) or more, 300 ppi or more, orfurthermore, 500 ppi or more.

Furthermore, according to one embodiment of the present invention, theaperture ratio can be improved even in a high-resolution display device,which makes it possible to use light from a light source such as abacklight efficiently, so that power consumption of the display devicecan be reduced.

Next, FIG. 20 is a cross-sectional view taken along dashed-dotted linesA-B and C-D in FIG. 19. The transistor 102 illustrated in FIG. 19 is achannel-etched transistor. Note that the transistor 102 in the channellength direction, a connection portion between the transistor 102 andthe conductive film 29 c functioning as a pixel electrode, and thecapacitor 105 are illustrated in the cross-sectional view taken alongdashed-dotted line A-B, and the transistor 102 in the channel widthdirection is illustrated in the cross-sectional view taken alongdashed-dotted line C-D.

Since the liquid crystal display device described in this embodiment isdriven in a VA mode, a liquid crystal element 322 a includes theconductive film 29 c functioning as a pixel electrode included in theelement layer of the first substrate 11, the conductive film 350included in the element layer of the second substrate 342, and theliquid crystal layer 320.

In addition, the transistor 102 in FIG. 20 has a structure similar tothat of the transistor 102 in Embodiment 1. The conductive film 29 cfunctioning as a pixel electrode connected to one of the conductivefilms 21 a and 21 b functioning as a source electrode and a drainelectrode (here, connected to the conductive film 21 b) is formed overthe nitride insulating film 27. In the opening 41 of the nitrideinsulating film 27, the conductive film 21 b is connected to theconductive film 29 c functioning as a pixel electrode.

The conductive film 29 c functioning as a pixel electrode can be formedusing as appropriate a material and a manufacturing method similar tothose of the common electrode 29 in Embodiment 2.

The capacitor 105 in FIG. 20 includes the film 19 d having conductivityformed over the oxide insulating film 17, the nitride insulating film27, and the conductive film 29 c functioning as a pixel electrode.

Over the transistor 102 in this embodiment, the oxide insulating films23 and 25 which are isolated from each other are formed. The oxideinsulating films 23 and 25 which are isolated from each other overlapwith the oxide semiconductor film 19 a.

In addition, the organic insulating film 31 overlapping with the oxidesemiconductor film 19 a is provided over the nitride insulating film 27.The organic insulating film 31 overlapping with the oxide semiconductorfilm 19 a is provided over the transistor 102, whereby the surface ofthe oxide semiconductor film 19 a can be made apart from the surface ofthe organic insulating film 31. Thus, the surface of the oxidesemiconductor film 19 a is not affected by the electric field ofpositively charged particles adsorbed on the surface of the organicinsulating film 31 and therefore the reliability of the transistor 102can be improved.

In the capacitor 105, the film 19 d having conductivity is differentfrom that in Embodiment 2 and is not connected to the conductive film 21b. In contrast, the film 19 d having conductivity is in contact with aconductive film 21 d. The conductive film 21 d functions as a capacitorline. The film 19 d having conductivity can be formed using a metaloxide film similar to that used for the pixel electrode 19 b inEmbodiment 2. In other words, the film 19 d having conductivity is ametal oxide film containing the same metal element as the oxidesemiconductor film 19 a. Moreover, a formation method similar to that ofthe pixel electrode 19 b in Embodiment 2 can be used as appropriate forthe film 19 d having conductivity.

Next, a method for manufacturing the transistor 102 and the capacitor105 in FIG. 20 is described with reference to FIGS. 21A to 21C and FIGS.22A to 22C.

A conductive film is formed over the first substrate 11 and then etchedusing a mask formed through a first photolithography process inEmbodiment 2, whereby the conductive film 13 functioning as a gateelectrode is formed over the first substrate 11 (see FIG. 21A).

Next, the nitride insulating film 15 and the oxide insulating film 16are formed over the first substrate 11 and the conductive film 13functioning as a gate electrode. Next, an oxide semiconductor film isformed over the oxide insulating film 16 and then etched using a maskformed through a second photolithography process in Embodiment 2,whereby the oxide semiconductor films 19 a and 19 c are formed (see FIG.21B).

Next, a conductive film is formed over the oxide insulating film 16 andthe oxide semiconductor films 19 a and 19 c and then etched using a maskformed through a third photolithography process in Embodiment 2, wherebythe conductive films 21 a, 21 b, and 21 d are formed (see FIG. 21C). Atthis time, the conductive film 21 b is formed so as not to be in contactwith the oxide semiconductor film 19 c. The conductive film 21 d isformed so as to be in contact with the oxide semiconductor film 19 c.

Next, an oxide insulating film is formed over the oxide insulating film16, the oxide semiconductor films 19 a and 19 c, and the conductivefilms 21 a, 21 b, and 21 d and then etched using a mask formed through afourth photolithography process in Embodiment 2, whereby the oxideinsulating films 23 and 25 are formed (see FIG. 22A).

Next, a nitride insulating film is formed over the oxide insulating film17, the oxide semiconductor films 19 a and 19 c, the conductive films 21a, 21 b, and 21 d, and the oxide insulating films 23 and 25 and thenetched using a mask formed through a fifth photolithography process inEmbodiment 2, whereby the nitride insulating film 27 having the opening41 through which part of the conductive film 21 b is exposed is formed(see FIG. 22B).

Through the above steps, the oxide semiconductor film 19 c becomes thefilm 19 d having conductivity. When a silicon nitride film is formedlater by a plasma CVD method as the nitride insulating film 27, hydrogencontained in the silicon nitride film is diffused to the oxidesemiconductor film 19 c; thus, the conductivity of the film 19 d havingconductivity can be enhanced.

Next, a conductive film is formed over the conductive film 21 b and thenitride insulating film 27 and then etched using a mask formed through asixth photolithography process in Embodiment 2, whereby the conductivefilm 29 c connected to the conductive film 21 b is formed (see FIG.22C).

From the above, as for a semiconductor device including an oxidesemiconductor film, a semiconductor device with improved electricalcharacteristics can be obtained.

On an element substrate of the semiconductor device described in thisembodiment, one electrode of the capacitor is formed at the same time asthe oxide semiconductor film of the transistor. In addition, theconductive film functioning as a pixel electrode is used as the otherelectrode of the capacitor. Thus, a step of forming another conductivefilm is not needed to form the capacitor, and the number of steps ofmanufacturing the display device can be reduced. Furthermore, since thepair of electrodes has a light-transmitting property, the capacitor hasa light-transmitting property. As a result, the area occupied by thecapacitor can be increased and the aperture ratio in a pixel can beincreased.

Modification Example 1

In this embodiment, a display device that can be manufactured with asmall number of masks as compared with that of the semiconductor devicedescribed in any of Embodiments 1 to 4 is described with reference toFIG. 23.

In the display device illustrated in FIG. 23, the number of masks can bereduced by not etching the oxide insulating film 22 and the oxideinsulating film 24 formed over the transistor 102. In addition, thenitride insulating film 27 is formed over the oxide insulating film 24,and an opening 41 a through which part of the conductive film 21 b isexposed is formed in the oxide insulating films 22 and 24 and thenitride insulating film 27. A conductive film 29 d functioning as apixel electrode, which is connected to the conductive film 21 b in theopening 41 a, is formed over the nitride insulating film 27.

The conductive film 21 d is formed over the oxide insulating film 17.Since the conductive film 21 d is formed at the same time as theconductive films 21 a and 21 b are formed, an additional photomask isnot needed to form the conductive film 21 d. The conductive film 21 dfunctions as a capacitor line. That is, a capacitor 105 a includes theconductive film 21 d, the oxide insulating film 22, the oxide insulatingfilm 24, the nitride insulating film 27, and the conductive film 29 dfunctioning as a pixel electrode.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments.

Embodiment 4

In this embodiment, a display device which is different from the displaydevices in Embodiment 2 and a manufacturing method thereof are describedwith reference to drawings. This embodiment is different from Embodiment2 in that the transistor has a structure in which an oxide semiconductorfilm is provided between different gate electrodes, that is, a dual-gatestructure. Note that the structures similar to those in Embodiment 2 arenot described repeatedly here.

A specific structure of an element layer formed over the first substrate11 included in the display device is described. The transistor providedin the display device of this embodiment is different from that inEmbodiment 2 in that a conductive film 29 b functioning as a gateelectrode and overlapping part of or the whole of each of the conductivefilm 13 functioning as a gate electrode, the oxide semiconductor film 19a, the conductive films 21 a and 21 b, and the oxide insulating film 25is provided. The conductive film 29 b functioning as a gate electrode isconnected to the conductive film 13 functioning as a gate electrode inthe opening 41 a, 41 b.

A transistor 102 a illustrated in FIG. 24 is a channel-etchedtransistor. Note that the transistor 102 a in the channel lengthdirection and a capacitor 105 are illustrated in a cross-sectional viewin a portion A-B, and the transistor 102 a in the channel widthdirection and a connection portion between the conductive film 13functioning as a gate electrode and the conductive film 29 b functioningas a gate electrode are illustrated in a cross-sectional view in aportion C-D.

The transistor 102 a in FIG. 24 has a dual-gate structure and includesthe conductive film 13 functioning as a gate electrode over the firstsubstrate 11. In addition, the transistor 102 a includes the nitrideinsulating film 15 formed over the first substrate 11 and the conductivefilm 13 functioning as a gate electrode, the oxide insulating film 17formed over the nitride insulating film 15, the oxide semiconductor film19 a overlapping with the conductive film 13 functioning as a gateelectrode with the nitride insulating film 15 and the oxide insulatingfilm 17 provided therebetween, and the conductive films 21 a and 21 bfunctioning as a source electrode and a drain electrode which are incontact with the oxide semiconductor film 19 a. Moreover, the oxideinsulating film 23 is formed over the oxide insulating film 17, theoxide semiconductor film 19 a, and the conductive films 21 a and 21 bfunctioning as a source electrode and a drain electrode, and the oxideinsulating film 25 is formed over the oxide insulating film 23. Thenitride insulating film 27 is formed over the nitride insulating film15, the oxide insulating film 23, the oxide insulating film 25, and theconductive film 21 b. The pixel electrode 19 b is formed over the oxideinsulating film 17. The pixel electrode 19 b is connected to one of theconductive films 21 a and 21 b functioning as a source electrode and adrain electrode, here, connected to the conductive film 21 b. The commonelectrode 29 and the conductive film 29 b functioning as a gateelectrode are formed over the nitride insulating film 27.

As illustrated in the cross-sectional view in a portion C-D, theconductive film 29 b functioning as a gate electrode is connected to theconductive film 13 functioning as a gate electrode in the opening 41 a,41 b provided in the nitride insulating film 15, the oxide insulatingfilm 17, the oxide insulating film 23, the oxide insulating film 25, andthe nitride insulating film 27. That is, the conductive film 13functioning as a gate electrode and the conductive film 29 b functioningas a gate electrode have the same potential.

Thus, by applying voltage at the same potential to each gate electrodeof the transistor 102 a, variation in the initial characteristics can bereduced, and degradation of the transistor 102 a after the −GBT stresstest and a change in the rising voltage of on-state current at differentdrain voltages can be suppressed. In addition, a region where carriersflow in the oxide semiconductor film 19 a becomes larger in the filmthickness direction, so that the amount of carrier movement isincreased. As a result, the on-state current of the transistor 102 a isincreased, and the field-effect mobility is increased. Typically, thefield-effect mobility is greater than or equal to 20 cm²/V·s.

Over the transistor 102 a in this embodiment, the oxide insulating films23 and 25 are formed. The oxide insulating films 23 and 25 overlap withthe oxide semiconductor film 19 a. In the cross-sectional view in thechannel width direction, end portions of the oxide insulating films 23and 25 are positioned on an outer side of an end portion of the oxidesemiconductor film 19 a. Furthermore, in the channel width direction inFIG. 24, the conductive film 29 b functioning as a gate electrode ispositioned at end portions of the oxide insulating films 23 and 25.

An end portion processed by etching or the like of the oxidesemiconductor film is damaged by processing, to produce defects and alsocontaminated by the attachment of an impurity, or the like. Thus, theend portion of the oxide semiconductor film is easily activated byapplication of a stress such as an electric field, thereby easilybecoming n-type (having a low resistance). Therefore, the end portion ofthe oxide semiconductor film 19 a overlapping with the conductive film13 functioning as a gate electrode easily becomes n-type. When the endportion which becomes n-type is provided between the conductive films 21a and 21 b functioning as a source electrode and a drain electrode, theregion which becomes n-type functions as a carrier path, resulting in aparasitic channel. However, as illustrated in the cross-sectional viewin a portion C-D, when the conductive film 29 b functioning as a gateelectrode faces a side surface of the oxide semiconductor film 19 a withthe oxide insulating films 23 and 25 provided therebetween in thechannel width direction, due to the electric field of the conductivefilm 29 b functioning as a gate electrode, generation of a parasiticchannel on the side surface of the oxide semiconductor film 19 a or in aregion including the side surface and the vicinity of the side surfaceis suppressed. As a result, a transistor which has excellent electricalcharacteristics such as a sharp increase in the drain current at thethreshold voltage is obtained.

In the capacitor 105 a, the pixel electrode 19 b is formed at the sametime as the oxide semiconductor film 19 a and has increased conductivityby containing an impurity. Alternatively, the pixel electrode 19 b isformed at the same time as the oxide semiconductor film 19 a, and hasincreased conductivity by containing oxygen vacancies generated byplasma damage or the like. Alternatively, the pixel electrode 19 b isformed at the same time as the oxide semiconductor film 19 a, and hasincreased conductivity by containing impurities and oxygen vacanciesgenerated by plasma damage or the like.

On an element substrate of the display device described in thisembodiment, the pixel electrode is formed at the same time as the oxidesemiconductor film of the transistor. The pixel electrode also functionsas one of electrodes of the capacitor. The common electrode alsofunctions as the other of electrodes of the capacitor. Thus, a step offorming another conductive film is not needed to form the capacitor, andthe number of steps of manufacturing the semiconductor device can bereduced. The capacitor has a light-transmitting property. As a result,the area occupied by the capacitor can be increased and the apertureratio in a pixel can be increased.

Details of the transistor 102 a are described below. Note that thecomponents with the same reference numerals as those in Embodiment 2 arenot described here.

The conductive film 29 b functioning as a gate electrode can be formedusing a material similar to that of the common electrode 29 inEmbodiment 2.

Next, a method for manufacturing the transistor 102 a and the capacitor105 a in FIG. 24 is described with reference to FIGS. 10A to 10D, FIGS.11A to 11D, FIG. 12A, and FIGS. 25A to 25C.

As in Embodiment 2, through the steps illustrated in FIGS. 10A to 12A,the conductive film 13 functioning as a gate electrode, the nitrideinsulating film 15, the oxide insulating film 16, the oxidesemiconductor film 19 a, the pixel electrode 19 b, the conductive films21 a and 21 b functioning as a source electrode and a drain electrode,the oxide insulating film 22, the oxide insulating film 24, and thenitride insulating film 26 are formed over the first substrate 11. Inthese steps, photography processes using the first photomask to thefourth photomask are performed.

Next, a mask is formed over the nitride insulating film 26 through aphotolithography process using a fifth photomask, and then part of thenitride insulating film 26 is etched using the mask; thus, the nitrideinsulating film 27 having the opening 41 a, 41 b is formed asillustrated in FIG. 25A.

Next, as illustrated in FIG. 25B, the conductive film 28 to be thecommon electrode 29 and the conductive film 29 b functioning as a gateelectrode is formed over the conductive film 13 functioning as a gateelectrode, and the nitride insulating film 27.

Then, a mask is formed over the conductive film 28 by a photolithographyprocess using a sixth photomask. Next, as illustrated in FIG. 25C, partof the conductive film 28 is etched with the use of the mask to form thecommon electrode 29 and the conductive film 29 b functioning as a gateelectrode. After that, the mask is removed.

Through the above process, the transistor 102 a is manufactured and thecapacitor 105 a can also be manufactured.

In the transistor described in this embodiment, when the conductive film29 b functioning as a gate electrode faces a side surface of the oxidesemiconductor film 19 a with the oxide insulating films 23 and 25provided therebetween in the channel width direction, due to theelectric field of the conductive film 29 b functioning as a gateelectrode, generation of a parasitic channel on the side surface of theoxide semiconductor film 19 a or in a region including the side surfaceand the vicinity of the side surface is suppressed. As a result, atransistor which has excellent electrical characteristics such as asharp increase in the drain current at the threshold voltage isobtained.

The element substrate of the display device of this embodiment isprovided with a common electrode including a stripe region extending ina direction intersecting with a signal line. Therefore, the displaydevice can have excellent contrast.

On an element substrate of the display device described in thisembodiment, the pixel electrode is formed at the same time as the oxidesemiconductor film of the transistor. The pixel electrode functions asthe one of electrodes of the capacitor. The common electrode alsofunctions as the other of electrodes of the capacitor. Thus, a step offorming another conductive film is not needed to form the capacitor, andthe number of steps of manufacturing the display device can be reduced.The capacitor has a light-transmitting property. As a result, the areaoccupied by the capacitor can be increased and the aperture ratio in apixel can be increased.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments.

Embodiment 5

In this embodiment, a display device including a transistor in which thenumber of defects in an oxide semiconductor film can be further reducedas compared with the above embodiments is described with reference todrawings. The transistor described in this embodiment is different fromany of the transistors in Embodiments 2 to 4 in that a multilayer filmincluding a plurality of oxide semiconductor films is provided. Here,details are described using the transistor in Embodiment 2.

FIGS. 26A and 26B each illustrate a cross-sectional view of an elementsubstrate included in a display device. FIGS. 26A and 26B arecross-sectional views taken along dashed-dotted lines A-B and C-D inFIG. 4.

A transistor 102 b in FIG. 26A includes a multilayer film 37 aoverlapping with the conductive film 13 functioning as a gate electrodewith the nitride insulating film 15 and the oxide insulating film 17provided therebetween, and the conductive films 21 a and 21 bfunctioning as a source electrode and a drain electrode in contact withthe multilayer film 37 a. The oxide insulating film 23, the oxideinsulating film 25, and the nitride insulating film 27 are formed overthe nitride insulating film 15, the oxide insulating film 17, themultilayer film 37 a, and the conductive films 21 a and 21 b functioningas a source electrode and a drain electrode.

The capacitor 105 b in FIG. 26A includes a multilayer film 37 b formedover the oxide insulating film 17, the nitride insulating film 27 incontact with the multilayer film 37 b, and the common electrode 29 incontact with the nitride insulating film 27. The multilayer film 37 bfunctions as a pixel electrode.

In the transistor 102 b described in this embodiment, the multilayerfilm 37 a includes the oxide semiconductor film 19 a and an oxidesemiconductor film 39 a. That is, the multilayer film 37 a has atwo-layer structure. In addition, part of the oxide semiconductor film19 a functions as a channel region. Moreover, the oxide insulating film23 is formed in contact with the multilayer film 37 a, and the oxideinsulating film 25 is formed in contact with the oxide insulating film23. That is, the oxide semiconductor film 39 a is provided between theoxide semiconductor film 19 a and the oxide insulating film 23.

The oxide semiconductor film 39 a is an oxide film containing one ormore elements that constitute the oxide semiconductor film 19 a. Thus,interface scattering is unlikely to occur at the interface between theoxide semiconductor films 19 a and 39 a. Thus, the transistor can havehigh field-effect mobility because the movement of carriers is nothindered at the interface.

The oxide semiconductor film 39 a is typically an In—Ga oxide film, anIn—Zn oxide film, or an In-M-Zn oxide film (M represents Al, Ga, Y, Zr,Sn, La, Ce, or Nd). The energy at the conduction band bottom of theoxide semiconductor film 39 a is closer to a vacuum level than that ofthe oxide semiconductor film 19 a is, and typically, the differencebetween the energy at the conduction band bottom of the oxidesemiconductor film 39 a and the energy at the conduction band bottom ofthe oxide semiconductor film 19 a is any one of 0.05 eV or more, 0.07 eVor more, 0.1 eV or more, or 0.15 eV or more, and any one of 2 eV orless, 1 eV or less, 0.5 eV or less, or 0.4 eV or less. That is, thedifference between the electron affinity of the oxide semiconductor film39 a and the electron affinity of the oxide semiconductor film 19 a isany one of 0.05 eV or more, 0.07 eV or more, 0.1 eV or more, or 0.15 eVor more, and any one of 2 eV or less, 1 eV or less, 0.5 eV or less, or0.4 eV or less.

The oxide semiconductor film 39 a preferably contains In because carriermobility (electron mobility) can be increased.

When the oxide semiconductor film 39 a contains a larger amount of Al,Ga, Y, Zr, Sn, La, Ce, or Nd in an atomic ratio than the amount of In inan atomic ratio, any of the following effects may be obtained: (1) theenergy gap of the oxide semiconductor film 39 a is widened; (2) theelectron affinity of the oxide semiconductor films film 39 a is reduced;(3) scattering of impurities from the outside is reduced; (4) aninsulating property increases as compared to the oxide semiconductorfilm 19 a; and (5) oxygen vacancies are less likely to be generatedbecause Al, Ga, Y, Zr, Sn, La, Ce, or Nd is a metal element stronglybonded to oxygen.

In the case where the oxide semiconductor film 39 a is an In-M-Zn oxidefilm, the proportions of In and M when the summation of In and M isassumed to be 100 atomic % are preferably as follows: the atomicpercentage of In is less than 50 atomic % and the atomic percentage of Mis more than 50 atomic %; further preferably, the atomic percentage ofIn is less than 25 atomic % and the atomic percentage of M is more than75 atomic %.

Furthermore, in the case where each of the oxide semiconductor films 19a and 39 a is an In-M-Zn oxide film (M represents Al, Ga, Y, Zr, Sn, La,Ce, or Nd), the proportion of M atoms (M represents Al, Ga, Y, Zr, Sn,La, Ce, or Nd) in the oxide semiconductor film 39 a is higher than thatin the oxide semiconductor film 19 a. As a typical example, theproportion of M in the oxide semiconductor film 39 a is 1.5 times ormore, preferably twice or more, further preferably three times or moreas high as that in the oxide semiconductor film 19 a.

Furthermore, in the case where each of the oxide semiconductor film 19 aand the oxide semiconductor film 39 a is an In-M-Zn oxide film (Mrepresents Al, Ga, Y, Zr, Sn, La, Ce, or Nd), when In:M:Zn=x₁:y₁:z₁[atomic ratio] is satisfied in the oxide semiconductor film 39 a andIn:M:Zn=x₂:y₂:z₂ [atomic ratio] is satisfied in the oxide semiconductorfilm 19 a, y₁/x₁ is higher than y₂/x₂. Preferably, y₁/x₁ is 1.5 times ormore as high as y₂/x₂. Further preferably, y₁/x₁ is twice or more ashigh as y₂/x₂. Still further preferably, y₁/x₁ is three times or more ashigh as y₂/x₂.

In the case where the oxide semiconductor film 19 a is an In-M-Zn oxidefilm (M is Al, Ga, Y, Zr, Sn, La, Ce, or Nd) and a target having theatomic ratio of metal elements of In:M:Zn=x₁:y₁:z₁ is used for formingthe oxide semiconductor film 19 a, x₁/y₁ is preferably greater than orequal to ⅓ and less than or equal to 6, further preferably greater thanor equal to 1 and less than or equal to 6, and z₁/y₁ is preferablygreater than or equal to ⅓ and less than or equal to 6, furtherpreferably greater than or equal to 1 and less than or equal to 6. Notethat when z₁/y₁ is greater than or equal to 1 and less than or equal to6, a CAAC-OS film to be described later as the oxide semiconductor film19 a is easily formed. Typical examples of the atomic ratio of the metalelements of the target are In:M:Zn=1:1:1, In:M:Zn=1:1:1.2, andIn:M:Zn=3:1:2.

In the case where the oxide semiconductor film 39 a is an In-M-Zn oxidefilm (M is Al, Ga, Y, Zr, Sn, La, Ce, or Nd) and a target having theatomic ratio of metal elements of In:M:Zn=x₂:y₂:z₂ is used for formingthe oxide semiconductor film 39 a, x₂/y₂ is preferably less than x₁/y₁,and z₂/y₂ is preferably greater than or equal to ⅓ and less than orequal to 6, further preferably greater than or equal to 1 and less thanor equal to 6. Note that when z₂/y₂ is greater than or equal to 1 andless than or equal to 6, a CAAC-OS film to be described later as theoxide semiconductor film 39 a is easily formed. Typical examples of theatomic ratio of the metal elements of the target are In:M:Zn=1:3:2,In:M:Zn=1:3:4, In:M:Zn=1:3:6, In:M:Zn=1:3:8, In:M:Zn=1:4:4,In:M:Zn=1:4:5, and In:M:Zn=1:6:8.

Note that the proportion of each metal element in the atomic ratio ofeach of the oxide semiconductor films 19 a and the oxide semiconductorfilm 39 a varies within a range of ±40% of that in the above atomicratio as an error.

The oxide semiconductor film 39 a also functions as a film that relievesdamage to the oxide semiconductor film 19 a at the time of forming theoxide insulating film 25 later.

The thickness of the oxide semiconductor film 39 a is greater than orequal to 3 nm and less than or equal to 100 nm, preferably greater thanor equal to 3 nm and less than or equal to 50 nm.

The oxide semiconductor film 39 a may have a non-single-crystalstructure, for example, like the oxide semiconductor film 19 a. Thenon-single-crystal structure includes a c-axis aligned crystalline oxidesemiconductor (CAAC-OS) which is described later, a polycrystallinestructure, a microcrystalline structure which is described later, or anamorphous structure, for example.

The oxide semiconductor film 39 a may have an amorphous structure, forexample. The oxide semiconductor films having the amorphous structureeach have disordered atomic arrangement and no crystalline component,for example.

Note that the oxide semiconductor films 19 a and 39 a may each be amixed film including two or more of the following: a region having anamorphous structure, a region having a microcrystalline structure, aregion having a polycrystalline structure, a CAAC-OS region, and aregion having a single-crystal structure. The mixed film has asingle-layer structure including, for example, two or more of a regionhaving an amorphous structure, a region having a microcrystallinestructure, a region having a polycrystalline structure, a CAAC-OSregion, and a region having a single-crystal structure in some cases.Furthermore, in some cases, the mixed film has a stacked-layer structurein which two or more of the following regions are stacked: a regionhaving an amorphous structure, a region having a microcrystallinestructure, a region having a polycrystalline structure, a CAAC-OSregion, and a region having a single-crystal structure.

Here, the oxide semiconductor film 39 a is formed between the oxidesemiconductor film 19 a and the oxide insulating film 23. Thus, ifcarrier traps are formed between the oxide semiconductor film 39 a andthe oxide insulating film 23 by impurities and defects, electronsflowing in the oxide semiconductor film 19 a are less likely to becaptured by the carrier traps because there is a distance between thecarrier traps and the oxide semiconductor film 19 a. Accordingly, theamount of on-state current of the transistor can be increased, and thefield-effect mobility can be increased. When the electrons are capturedby the carrier traps, the electrons become negative fixed charges. As aresult, a threshold voltage of the transistor changes. However, by thedistance between the oxide semiconductor film 19 a and the carriertraps, capture of electrons by the carrier traps can be reduced, andaccordingly, the amount of change in the threshold voltage can bereduced.

Impurities from the outside can be blocked by the oxide semiconductorfilm 39 a, and accordingly, the amount of impurities that aretransferred from the outside to the oxide semiconductor film 19 a can bereduced. Furthermore, an oxygen vacancy is less likely to be formed inthe oxide semiconductor film 39 a. Consequently, the impurityconcentration and the number of oxygen vacancies in the oxidesemiconductor film 19 a can be reduced.

Note that the oxide semiconductor films 19 a and 39 a are not onlyformed by simply stacking each film, but also are formed to have acontinuous junction (here, in particular, a structure in which theenergy of the bottom of the conduction band is changed continuouslybetween each film). In other words, a stacked-layer structure in whichthere exist no impurity that forms a defect level such as a trap centeror a recombination center at the interface between the films isprovided. If an impurity exists between the oxide semiconductor films 19a and 39 a which are stacked, a continuity of the energy band isdamaged, and the carrier is captured or recombined at the interface andthen disappears.

In order to form such a continuous energy band, it is necessary to formfilms continuously without being exposed to air, with use of amulti-chamber deposition apparatus (sputtering apparatus) including aload lock chamber. Each chamber in the sputtering apparatus ispreferably evacuated to be a high vacuum state (to the degree of about5×10⁻⁷ Pa to 1×10⁻⁴ Pa) with an adsorption vacuum evacuation pump suchas a cryopump in order to remove water or the like, which serves as animpurity against the oxide semiconductor film, as much as possible.Alternatively, a turbo molecular pump and a cold trap are preferablycombined so as to prevent a backflow of a gas, especially a gascontaining carbon or hydrogen from an exhaust system to the inside ofthe chamber.

As in a transistor 102 c in FIG. 26B, a multilayer film 38 a may beprovided instead of the multilayer film 37 a.

In addition, as in a capacitor 105 c in FIG. 26B, a multilayer film 38 bmay be provided instead of the multilayer film 37 b.

The multilayer film 38 a includes an oxide semiconductor film 49 a, theoxide semiconductor film 19 a, and the oxide semiconductor film 39 a.That is, the multilayer film 38 a has a three-layer structure.Furthermore, the oxide semiconductor film 19 a functions as a channelregion.

The oxide semiconductor film 49 a can be formed using a material and aformation method similar to those of the oxide semiconductor film 39 a.

The multilayer film 38 b includes an oxide semiconductor film 49 b, anoxide semiconductor film 19 f, and an oxide semiconductor film 39 b. Inother words, the multilayer film 38 b has a three-layer structure. Themultilayer film 38 b functions as a pixel electrode.

The oxide semiconductor film 19 f can be formed using a material and aformation method similar to those of the pixel electrode 19 b asappropriate. The oxide semiconductor film 49 b can be formed using amaterial and a formation method similar to those of the oxidesemiconductor film 39 b as appropriate.

In addition, the oxide insulating film 17 and the oxide semiconductorfilm 49 a are in contact with each other. That is, the oxidesemiconductor film 49 a is provided between the oxide insulating film 17and the oxide semiconductor film 19 a.

The multilayer film 38 a and the oxide insulating film 23 are in contactwith each other. In addition, the oxide semiconductor film 39 a and theoxide insulating film 23 are in contact with each other. That is, theoxide semiconductor film 39 a is provided between the oxidesemiconductor film 19 a and the oxide insulating film 23.

It is preferable that the thickness of the oxide semiconductor film 49 abe smaller than that of the oxide semiconductor film 19 a. When thethickness of the oxide semiconductor film 49 a is greater than or equalto 1 nm and less than or equal to 5 nm, preferably greater than or equalto 1 nm and less than or equal to 3 nm, the amount of change in thethreshold voltage of the transistor can be reduced.

In the transistor described in this embodiment, the oxide semiconductorfilm 39 a is provided between the oxide semiconductor film 19 a and theoxide insulating film 23. Thus, if carrier traps are formed between theoxide semiconductor film 39 a and the oxide insulating film 23 byimpurities and defects, electrons flowing in the oxide semiconductorfilm 19 a are less likely to be captured by the carrier traps becausethere is a distance between the carrier traps and the oxidesemiconductor film 19 a. Accordingly, the amount of on-state current ofthe transistor can be increased, and the field-effect mobility can beincreased. When the electrons are captured by the carrier traps, theelectrons become negative fixed charges. As a result, a thresholdvoltage of the transistor changes. However, by the distance between theoxide semiconductor film 19 a and the carrier traps, capture ofelectrons by the carrier traps can be reduced, and accordingly, theamount of change in the threshold voltage can be reduced.

Impurities from the outside can be blocked by the oxide semiconductorfilm 39 a, and accordingly, the amount of impurities that aretransferred from the outside to the oxide semiconductor film 19 a can bereduced. Furthermore, an oxygen vacancy is less likely to be formed inthe oxide semiconductor film 39 a. Consequently, the impurityconcentration and the number of oxygen vacancies in the oxidesemiconductor film 19 a can be reduced.

Furthermore, the oxide semiconductor film 49 a is provided between theoxide insulating film 17 and the oxide semiconductor film 19 a, and theoxide semiconductor film 39 a is provided between the oxidesemiconductor film 19 a and the oxide insulating film 23. Thus, it ispossible to reduce the concentration of silicon or carbon in thevicinity of the interface between the oxide semiconductor film 49 a andthe oxide semiconductor film 19 a, the concentration of silicon orcarbon in the oxide semiconductor film 19 a, or the concentration ofsilicon or carbon in the vicinity of the interface between the oxidesemiconductor film 39 a and the oxide semiconductor film 19 a.Consequently, in the multilayer film 38 a, the absorption coefficientderived from a constant photocurrent method is lower than 1×10⁻³/cm,preferably lower than 1×10⁻⁴/cm, and thus density of localized levels isextremely low.

The transistor 102 c having such a structure includes very few defectsin the multilayer film 38 a including the oxide semiconductor film 19 a;thus, the electrical characteristics of the transistor can be improved,and typically, the on-state current can be increased and thefield-effect mobility can be improved. Moreover, in a BT stress test anda BT photostress test which are examples of a stress test, the amount ofchange in threshold voltage is small, and thus, reliability is high.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments.

Embodiment 6

In this embodiment, one embodiment that can be applied to the oxidesemiconductor film in the transistor included in the display devicedescribed in the above embodiment is described.

An oxide semiconductor film is classified into, for example, anon-single-crystal oxide semiconductor film and a single crystal oxidesemiconductor film. Alternatively, an oxide semiconductor is classifiedinto, for example, a crystalline oxide semiconductor and an amorphousoxide semiconductor.

Examples of a non-single-crystal oxide semiconductor include a c-axisaligned crystalline oxide semiconductor (CAAC-OS), a polycrystallineoxide semiconductor, a microcrystalline oxide semiconductor, and anamorphous oxide semiconductor. In addition, examples of a crystallineoxide semiconductor include a single crystal oxide semiconductor, aCAAC-OS, a polycrystalline oxide semiconductor, and a microcrystallineoxide semiconductor.

The oxide semiconductor film may include one or more of the following:an oxide semiconductor having a single-crystal structure (hereinafterreferred to as a single-crystal oxide semiconductor); an oxidesemiconductor having a polycrystalline structure (hereinafter referredto as a polycrystalline oxide semiconductor); an oxide semiconductorhaving a microcrystalline structure (hereinafter referred to as amicrocrystalline oxide semiconductor); and an oxide semiconductor havingan amorphous structure (hereinafter referred to as an amorphous oxidesemiconductor). Furthermore, the oxide semiconductor film may include aCAAC-OS film. Furthermore, the oxide semiconductor film may include anamorphous oxide semiconductor and an oxide semiconductor having acrystal grain. Described below are a CAAC-OS and a microcrystallineoxide semiconductor as typical examples.

<CAAC-OS>

First, a CAAC-OS film is described.

The CAAC-OS film is one of oxide semiconductor films having a pluralityof c-axis aligned crystal parts.

With a transmission electron microscope (TEM), a combined analysis image(also referred to as a high-resolution TEM image) of a bright-fieldimage and a diffraction pattern of the CAAC-OS film is observed.Consequently, a plurality of crystal parts are observed clearly.However, a boundary between crystal parts, that is, a grain boundary isnot clearly observed even in the high-resolution TEM image. Thus, in theCAAC-OS film, a reduction in electron mobility due to the grain boundaryis less likely to occur.

According to the high-resolution cross-sectional TEM image of theCAAC-OS film observed in a direction substantially parallel to thesample surface, metal atoms are arranged in a layered manner in thecrystal parts. Each metal atom layer has a morphology that reflects asurface over which the CAAC-OS film is formed (hereinafter, a surfaceover which the CAAC-OS film is formed is referred to as a formationsurface) or a top surface of the CAAC-OS film, and is arranged parallelto the formation surface or the top surface of the CAAC-OS film.

On the other hand, according to the high-resolution planar TEM image ofthe CAAC-OS film observed in a direction substantially perpendicular tothe sample surface, metal atoms are arranged in a triangular orhexagonal configuration in the crystal parts. However, there is noregularity of arrangement of metal atoms between different crystalparts.

FIG. 27A is a high-resolution cross-sectional TEM image of a CAAC-OSfilm. FIG. 27B is a high-resolution cross-sectional TEM image obtainedby enlarging the image of FIG. 27A. In FIG. 27B, atomic arrangement ishighlighted for easy understanding.

FIG. 27C is Fourier transform images of regions each surrounded by acircle (the diameter is about 4 nm) between A and O and between O and A′in FIG. 27A. C-axis alignment can be observed in each region in FIG.27C. The c-axis direction between A and O is different from that betweenO and A′, which indicates that a grain in the region between A and O isdifferent from that between O and A′. In addition, between A and O, theangle of the c-axis continuously and gradually changes from 14.3°, 16.6°to 26.4°. Similarly, between O and A′, the angle of the c-axiscontinuously changes from −18.3°, −17.6°, to −15.9°.

Note that in an electron diffraction pattern of the CAAC-OS film, spots(bright spots) having alignment are shown. Meanwhile, spots are shown ina nanobeam electron diffraction pattern of the top surface of theCAAC-OS film obtained by using an electron beam having a probe diameterranging from 1 nm to 30 nm, for example (see FIG. 28A).

From the results of the high-resolution cross-sectional TEM image andthe high-resolution plan TEM image, alignment is found in the crystalparts in the CAAC-OS film.

Most of the crystal parts included in the CAAC-OS film each fit inside acube whose one side is less than 100 nm. Thus, there is a case where acrystal part included in the CAAC-OS film fits inside a cube whose oneside is less than 10 nm, less than 5 nm, or less than 3 nm. Note thatwhen a plurality of crystal parts included in the CAAC-OS film areconnected to each other, one large crystal region is formed in somecases. For example, a crystal region with an area of 2500 nm or more, 5μm² or more, or 1000 μm² or more is observed in some cases in the planhigh-resolution TEM image.

A CAAC-OS film is subjected to structural analysis with an X-raydiffraction (XRD) apparatus. For example, when the CAAC-OS filmincluding an InGaZnO₄ crystal is analyzed by an out-of-plane method, apeak appears frequently when the diffraction angle (2θ) is around 31°.This peak is derived from the (009) plane of the InGaZnO₄ crystal, whichindicates that crystals in the CAAC-OS film have c-axis alignment, andthat the c-axes are aligned in a direction substantially perpendicularto the formation surface or the top surface of the CAAC-OS film.

On the other hand, when the CAAC-OS film is analyzed by an in-planemethod in which an X-ray enters a sample in a direction substantiallyperpendicular to the c-axis, a peak appears frequently when 2θ is around56°. This peak is derived from the (110) plane of the InGaZnO₄ crystal.Here, analysis (φ scan) is performed under conditions where the sampleis rotated around a normal vector of a sample surface as an axis (φaxis) with 2θ fixed at around 56°. In the case where the sample is asingle-crystal oxide semiconductor film of InGaZnO₄, six peaks appear.The six peaks are derived from crystal planes equivalent to the (110)plane. On the other hand, in the case of a CAAC-OS film, a peak is notclearly observed even when φ scan is performed with 2θ fixed at around56°.

According to the above results, in the CAAC-OS film having c-axisalignment, while the directions of a-axes and b-axes are differentbetween crystal parts, the c-axes are aligned in a direction parallel toa normal vector of a formation surface or a normal vector of a topsurface. Thus, each metal atom layer arranged in a layered mannerobserved in the high-resolution cross-sectional TEM image corresponds toa plane parallel to the a-b plane of the crystal.

Note that the crystal part is formed concurrently with deposition of theCAAC-OS film or is formed through crystallization treatment such as heattreatment. As described above, the c-axis of the crystal is aligned in adirection parallel to a normal vector of a formation surface or a normalvector of a top surface. Thus, for example, in the case where a shape ofthe CAAC-OS film is changed by etching or the like, the c-axis might notbe necessarily parallel to a normal vector of a formation surface or anormal vector of a top surface of the CAAC-OS film.

Furthermore, distribution of c-axis aligned crystal parts in the CAAC-OSfilm is not necessarily uniform. For example, in the case where crystalgrowth leading to the crystal parts of the CAAC-OS film occurs from thevicinity of the top surface of the film, the proportion of the c-axisaligned crystal parts in the vicinity of the top surface is higher thanthat in the vicinity of the formation surface in some cases.Furthermore, when an impurity is added to the CAAC-OS film, a region towhich the impurity is added is altered, and the proportion of the c-axisaligned crystal parts in the CAAC-OS film varies depending on regions,in some cases.

Note that when the CAAC-OS film with an InGaZnO₄ crystal is analyzed byan out-of-plane method, a peak of 2θ may also be observed at around 36°,in addition to the peak of 2θ at around 31°. The peak of 2θ at around36° indicates that a crystal having no c-axis alignment is included inpart of the CAAC-OS film. It is preferable that in the CAAC-OS film, apeak of 2θ appear at around 31° and a peak of 2θ not appear at around36°.

The CAAC-OS film is an oxide semiconductor film having low impurityconcentration. The impurity is an element other than the main componentsof the oxide semiconductor film, such as hydrogen, carbon, silicon, or atransition metal element. In particular, an element that has higherbonding strength to oxygen than a metal element included in the oxidesemiconductor film, such as silicon, disturbs the atomic arrangement ofthe oxide semiconductor film by depriving the oxide semiconductor filmof oxygen and causes a decrease in crystallinity. In addition, a heavymetal such as iron or nickel, argon, carbon dioxide, or the like has alarge atomic radius (molecular radius), and thus disturbs the atomicarrangement of the oxide semiconductor film and causes a decrease incrystallinity when it is contained in the oxide semiconductor film. Notethat the impurity contained in the oxide semiconductor film mightfunction as a carrier trap or a carrier generation source.

The CAAC-OS film is an oxide semiconductor film having a low density ofdefect states. In some cases, oxygen vacancies in the oxidesemiconductor film function as carrier traps or function as carriergeneration sources when hydrogen is captured therein.

The state in which impurity concentration is low and density of defectstates is low (the amount of oxygen vacancies is small) is referred toas a “highly purified intrinsic” or “substantially highly purifiedintrinsic” state. A highly purified intrinsic or substantially highlypurified intrinsic oxide semiconductor film has few carrier generationsources, and thus can have a low carrier density. Thus, a transistorincluding the oxide semiconductor film rarely has negative thresholdvoltage (is rarely normally on). The highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor film has alow density of defect states, and thus has few carrier traps.Accordingly, the transistor including the oxide semiconductor film haslittle variation in electrical characteristics and high reliability.Electric charge trapped by the carrier traps in the oxide semiconductorfilm takes a long time to be released, and might behave like fixedelectric charge. Thus, the transistor which includes the oxidesemiconductor film having high impurity concentration and a high densityof defect states has unstable electrical characteristics in some cases.

With the use of the CAAC-OS film in a transistor, variation in theelectrical characteristics of the transistor due to irradiation withvisible light or ultraviolet light is small.

<Microcrystalline Oxide Semiconductor>

Next, a microcrystalline oxide semiconductor film is described.

A microcrystalline oxide semiconductor film has a region where a crystalpart is observed in a high resolution TEM image and a region where acrystal part is not clearly observed in a high resolution TEM image. Inmost cases, a crystal part in the microcrystalline oxide semiconductorfilm is greater than or equal to 1 nm and less than or equal to 100 nm,or greater than or equal to 1 nm and less than or equal to 10 nm. Amicrocrystal with a size greater than or equal to 1 nm and less than orequal to 10 nm, or a size greater than or equal to 1 nm and less than orequal to 3 nm is specifically referred to as nanocrystal (nc). An oxidesemiconductor film including nanocrystal is referred to as an nc-OS(nanocrystalline oxide semiconductor) film. In an image of the nc-OSfilm observed with a TEM, for example, a grain boundary is not easilyand clearly observed in some cases.

In the nc-OS film, a microscopic region (e.g., a region with a sizegreater than or equal to 1 nm and less than or equal to 10 nm, inparticular, a region with a size greater than or equal to 1 nm and lessthan or equal to 3 nm) has a periodic atomic order. Note that there isno regularity of crystal orientation between different crystal parts inthe nc-OS film. Thus, the orientation of the whole film is not observed.Accordingly, in some cases, the nc-OS film cannot be distinguished froman amorphous oxide semiconductor depending on an analysis method. Forexample, when the nc-OS film is subjected to structural analysis by anout-of-plane method with an XRD apparatus using an X-ray having adiameter larger than that of a crystal part, a peak which shows acrystal plane does not appear. Furthermore, a halo pattern is shown in aselected-area electron diffraction pattern of the nc-OS film obtained byusing an electron beam having a probe diameter (e.g., larger than orequal to 50 nm) larger than a diameter of a crystal part. Meanwhile,spots are shown in a nanobeam electron diffraction pattern of the nc-OSfilm obtained by using an electron beam having a probe diameter closeto, or smaller than the diameter of a crystal part. Furthermore, in ananobeam electron diffraction pattern of the nc-OS film, regions withhigh luminance in a circular (ring) pattern are observed in some cases.Moreover, in a nanobeam electron diffraction pattern of the nc-OS film,a plurality of spots are shown in a ring-like region in some cases (seeFIG. 28B).

The nc-OS film is an oxide semiconductor film that has high regularityas compared to an amorphous oxide semiconductor film. Therefore, thenc-OS film has a lower density of defect states than an amorphous oxidesemiconductor film. However, there is no regularity of crystalorientation between different crystal parts in the nc-OS film; hence,the nc-OS film has a higher density of defect states than the CAAC-OSfilm.

Next, an amorphous oxide semiconductor film is described.

The amorphous oxide semiconductor film has disordered atomic arrangementand no crystal part. For example, the amorphous oxide semiconductor filmdoes not have a specific state as in quartz.

In the high-resolution TEM image of the amorphous oxide semiconductorfilm, crystal parts cannot be found.

When the amorphous oxide semiconductor film is subjected to structuralanalysis by an out-of-plane method with an XRD apparatus, a peak whichshows a crystal plane does not appear. A halo pattern is shown in anelectron diffraction pattern of the amorphous oxide semiconductor film.Furthermore, a halo pattern is shown but a spot is not shown in ananobeam electron diffraction pattern of the amorphous oxidesemiconductor film.

The amorphous oxide semiconductor film contains impurities such ashydrogen at a high concentration. In addition, the amorphous oxidesemiconductor film has a high density of defect states.

The oxide semiconductor film having a high impurity concentration and ahigh density of defect states has many carrier traps or many carriergeneration sources.

Accordingly, the amorphous oxide semiconductor film has a much highercarrier density than the nc-OS film. Therefore, a transistor includingthe amorphous oxide semiconductor film tends to be normally on. Thus, insome cases, such an amorphous oxide semiconductor film can be applied toa transistor which needs to be normally on. Since the amorphous oxidesemiconductor film has a high density of defect states, carrier trapsmight be increased. Consequently, a transistor including the amorphousoxide semiconductor film has larger variation in electricalcharacteristics and lower reliability than a transistor including theCAAC-OS film or the nc-OS film.

Note that an oxide semiconductor film may have a structure havingphysical properties between the nc-OS film and the amorphous oxidesemiconductor film. The oxide semiconductor film having such a structureis specifically referred to as an amorphous-like oxide semiconductor(amorphous-like OS) film.

In a high-resolution TEM image of the amorphous-like OS film, a void maybe observed. Furthermore, in the high-resolution TEM image, there are aregion where a crystal part is clearly observed and a region where acrystal part is not observed. In the amorphous-like OS film,crystallization by a slight amount of electron beam used for TEMobservation occurs and growth of the crystal part is found sometimes. Incontrast, crystallization by a slight amount of electron beam used forTEM observation is less observed in the nc-OS film having good quality.

Note that the crystal part size in the amorphous-like OS film and thenc-OS film can be measured using high-resolution TEM images. Forexample, an InGaZnO₄ crystal has a layered structure in which twoGa—Zn—O layers are included between In—O layers. A unit cell of theInGaZnO₄ crystal has a structure in which nine layers of three In—Olayers and six Ga—Zn—O layers are layered in the c-axis direction.Accordingly, the spacing between these adjacent layers is equivalent tothe lattice spacing on the (009) plane (also referred to as d value).The value is calculated to 0.29 nm from crystal structure analysis.Thus, each of the lattice fringes in which the spacing therebetween isfrom 0.28 nm to 0.30 nm is regarded to correspond to the a-b plane ofthe InGaZnO₄ crystal, focusing on the lattice fringes in thehigh-resolution TEM image. Let the maximum length in the region in whichthe lattice fringes are observed be the size of crystal part of theamorphous-like OS film and the nc-OS film. Note that the crystal partwhose size is 0.8 nm or larger is selectively evaluated.

FIG. 35 shows examination results of change in average size of crystalparts (20-40 points) in the amorphous-like OS film and the nc-OS filmusing the high-resolution TEM images. As in FIG. 35, the crystal partsize in the amorphous-like OS film increases with an increase of thetotal amount of electron irradiation. Specifically, the crystal part ofapproximately 1.2 nm at the start of TEM observation grows to a size ofapproximately 2.6 nm at the total amount of electron irradiation of4.2×10⁸ e⁻/nm². In contrast, the crystal part size in the good-qualitync-OS film shows a little change from the start of electron irradiationto the total amount of electron irradiation of 4.2×10⁸ e⁻/nm² regardlessof the amount of electron irradiation.

Furthermore, in FIG. 35, by linear approximation of the change in thecrystal part size in the amorphous-like OS film and the nc-OS film andextrapolation to the total amount of electron irradiation of 0 e⁻/nm²,the average size of the crystal part is found to be a positive value.This means that the crystal parts exist in the amorphous-like OS filmand the nc-OS film before TEM observation.

Note that an oxide semiconductor film may be a stacked film includingtwo or more kinds of an amorphous oxide semiconductor film, amicrocrystalline oxide semiconductor film, and a CAAC-OS film, forexample.

In the case where the oxide semiconductor film has a plurality ofstructures, the structures can be analyzed using nanobeam electrondiffraction in some cases.

FIG. 28C illustrates a transmission electron diffraction measurementapparatus which includes an electron gun chamber 70, an optical system72 below the electron gun chamber 70, a sample chamber 74 below theoptical system 72, an optical system 76 below the sample chamber 74, anobservation chamber 80 below the optical system 76, a camera 78installed in the observation chamber 80, and a film chamber 82 below theobservation chamber 80. The camera 78 is provided to face toward theinside of the observation chamber 80. Note that the film chamber 82 isnot necessarily provided.

FIG. 28D illustrates an internal structure of the transmission electrondiffraction measurement apparatus illustrated in FIG. 28C. In thetransmission electron diffraction measurement apparatus, a substance 88which is positioned in the sample chamber 74 is irradiated withelectrons emitted from an electron gun installed in the electron gunchamber 70 through the optical system 72. The electrons which havepassed through the substance 88 enter a fluorescent plate 92 which isinstalled in the observation chamber 80 through the optical system 76. Apattern which depends on the intensity of the incident electrons appearsin the fluorescent plate 92, so that the transmitted electrondiffraction pattern can be measured.

The camera 78 is installed so as to face the fluorescent plate 92 andcan take a picture of a pattern appearing in the fluorescent plate 92.An angle formed by a straight line which passes through the center of alens of the camera 78 and the center of the fluorescent plate 92 and anupper surface of the fluorescent plate 92 is, for example, 15° or moreand 80° or less, 30° or more and 75° or less, or 45° or more and 70° orless. As the angle is reduced, distortion of the transmission electrondiffraction pattern taken by the camera 78 becomes larger. Note that ifthe angle is obtained in advance, the distortion of an obtainedtransmission electron diffraction pattern can be corrected. Note thatthe film chamber 82 may be provided with the camera 78. For example, thecamera 78 may be set in the film chamber 82 so as to be opposite to theincident direction of electrons 84. In this case, a transmissionelectron diffraction pattern with less distortion can be taken from therear surface of the fluorescent plate 92.

A holder for fixing the substance 88 that is a sample is provided in thesample chamber 74. The holder transmits electrons passing through thesubstance 88. The holder may have, for example, a function of moving thesubstance 88 in the direction of the X, Y, and Z axes. The movementfunction of the holder may have an accuracy of moving the substance inthe range of, for example, 1 nm to 10 nm, 5 nm to 50 nm, 10 nm to 100nm, 50 nm to 500 nm, and 100 nm to 1 μm. The range is preferablydetermined to be an optimal range for the structure of the substance 88.

Then, a method for measuring a transmission electron diffraction patternof a substance by the transmission electron diffraction measurementapparatus described above is described.

For example, changes in the structure of a substance can be observed bychanging (scanning) the irradiation position of the electrons 84 thatare a nanobeam in the substance, as illustrated in FIG. 28D. At thistime, when the substance 88 is a CAAC-OS film, a diffraction patternshown in FIG. 28A can be observed. When the substance 88 is an nc-OSfilm, a diffraction pattern shown in FIG. 28B can be observed.

Even when the substance 88 is a CAAC-OS film, a diffraction patternsimilar to that of an nc-OS film or the like is partly observed in somecases. Therefore, whether or not a CAAC-OS film is favorable can bedetermined by the proportion of a region where a diffraction pattern ofa CAAC-OS film is observed in a predetermined area (also referred to asproportion of CAAC). In the case of a high quality CAAC-OS film, forexample, the proportion of CAAC is higher than or equal to 50%,preferably higher than or equal to 80%, further preferably higher thanor equal to 90%, still further preferably higher than or equal to 95%.Note that a region where a diffraction pattern different from that of aCAAC-OS film is observed is referred to as the proportion of non-CAAC.

For example, transmission electron diffraction patterns were obtained byscanning a top surface of a sample including a CAAC-OS film obtainedjust after deposition (represented as “as-sputtered”) and a top surfaceof a sample including a CAAC-OS subjected to heat treatment at 450° C.in an atmosphere containing oxygen. Here, the proportion of CAAC wasobtained in such a manner that diffraction patterns were observed byscanning for 60 seconds at a rate of 5 nm/second and the obtaineddiffraction patterns were converted into still images every 0.5 seconds.Note that as an electron beam, a nanobeam with a probe diameter of 1 nmwas used. The above measurement was performed on six samples. Theproportion of CAAC was calculated using the average value of the sixsamples.

FIG. 29A shows the proportion of CAAC in each sample. The proportion ofCAAC of the CAAC-OS film obtained just after the deposition was 75.7%(the proportion of non-CAAC was 24.3%). The proportion of CAAC of theCAAC-OS film subjected to the heat treatment at 450° C. was 85.3% (theproportion of non-CAAC was 14.7%). These results show that theproportion of CAAC obtained after the heat treatment at 450° C. ishigher than that obtained just after the deposition. That is, heattreatment at a high temperature (e.g., higher than or equal to 400° C.)reduces the proportion of non-CAAC (increases the proportion of CAAC).Furthermore, the above results also indicate that even when thetemperature of the heat treatment is lower than 500° C., the CAAC-OSfilm can have a high proportion of CAAC.

Here, most of diffraction patterns different from that of a CAAC-OS filmare diffraction patterns similar to that of an nc-OS film. Furthermore,an amorphous oxide semiconductor film was not able to be observed in themeasurement region. Therefore, the above results suggest that the regionhaving a structure similar to that of an nc-OS film is rearranged by theheat treatment owing to the influence of the structure of the adjacentregion, whereby the region becomes CAAC.

FIGS. 29B and 29C are high-resolution planar TEM images of the CAAC-OSfilm obtained just after the deposition and the CAAC-OS film subjectedto the heat treatment at 450° C., respectively. Comparison between FIGS.29B and 29C shows that the CAAC-OS film subjected to the heat treatmentat 450° C. has more uniform film quality. That is, the heat treatment ata high temperature improves the film quality of the CAAC-OS film.

With such a measurement method, the structure of an oxide semiconductorfilm having a plurality of structures can be analyzed in some cases.

<Oxide Semiconductor Film and Oxide Conductor Film>

Next, the temperature dependence of conductivity of a film formed withan oxide semiconductor (hereinafter referred to as an oxidesemiconductor film (OS)) and that of a film formed with an oxideconductor (hereinafter referred to as an oxide conductor film (OC)),which can be used for the pixel electrode 19 b, is described withreference to FIG. 34. In FIG. 34, the horizontal axes representmeasurement temperature (the lower horizontal axis represents 1/T andthe upper horizontal axis represents T), and the vertical axisrepresents conductivity (1/p). Measurement results of the oxidesemiconductor film (OS) are plotted as triangles, and measurementresults of the oxide conductor film (OC) are plotted as circles.

Note that a sample including the oxide semiconductor film (OS) wasprepared by forming a 35-nm-thick In—Ga—Zn oxide film over a glasssubstrate by a sputtering method using a sputtering target with anatomic ratio of In:Ga:Zn=1:1:1.2, forming a 20-nm-thick In—Ga—Zn oxidefilm over the 35-nm-thick In—Ga—Zn oxide film by a sputtering methodusing a sputtering target with an atomic ratio of In:Ga:Zn=1:4:5,performing heat treatment at 450° C. in a nitrogen atmosphere and thenperforming heat treatment at 450° C. in an atmosphere of a mixed gas ofnitrogen and oxygen, and forming a silicon oxynitride film over theoxide films by a plasma CVD method.

A sample including the oxide conductor film (OC) is prepared by forminga 100-nm-thick In—Ga—Zn oxide film over a glass substrate by asputtering method using a sputtering target with an atomic ratio ofIn:Ga:Zn=1:1:1, performing heat treatment at 450° C. in a nitrogenatmosphere and then performing heat treatment at 450° C. in anatmosphere of a mixed gas of nitrogen and oxygen, and forming a siliconnitride film over the oxide film by a plasma CVD method.

As can be seen from FIG. 34, the temperature dependence of conductivityof the oxide conductor film (OC) is lower than the temperaturedependence of conductivity of the oxide semiconductor film (OS).Typically, the range of variation of conductivity of the oxide conductorfilm (OC) at temperatures from 80 K to 290 K is from more than −20% toless than +20%. Alternatively, the range of variation of conductivity attemperatures from 150 K to 250 K is from more than −10% to less than+10%. In other words, the oxide conductor is a degenerate semiconductorand it is suggested that the conduction band edge agrees with orsubstantially agrees with the Fermi level. Therefore, the oxideconductor film (OC) can be used for a resistor, a wiring, an electrode,a pixel electrode, a common electrode, or the like.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments.

Embodiment 7

In the transistor using an oxide semiconductor film, the current in anoff state (off-state current) can be made low, as described inEmbodiment 2. Accordingly, an electric signal such as a video signal canbe held for a longer period and a writing interval can be set longer.

With the use of a transistor with a low off-state current, the liquidcrystal display device in this embodiment can display images by at leasttwo driving methods (modes). The first driving mode is a conventionaldriving method of a liquid crystal display device, in which data isrewritten sequentially every frame. The second driving mode is a drivingmethod in which data rewriting is stopped after data writing isexecuted, i.e., a driving mode with a reduced refresh rate.

Moving images are displayed in the first driving mode. A still image canbe displayed without change in image data every frame; thus, it is notnecessary to rewrite data every frame. When the liquid crystal displaydevice is driven in the second driving mode in displaying still images,power consumption can be reduced with fewer screen flickers.

A liquid crystal element used in the liquid crystal display device inthis embodiment has a large-area capacitor that can accumulate a largecapacitance. Thus, it is possible to make the retention period ofpotentials on the pixel electrode longer and to apply such a drivingmode with a reduced refresh rate. In addition, a change in voltageapplied to the liquid crystal layer can be suppressed for a long timeeven when the liquid crystal display device is used in the driving modewith a reduced refresh rate. This makes it possible to prevent screenflickers from being perceived by a user more effectively. Accordingly,the power consumption can be reduced and the display quality can beimproved.

An effect of reducing the refresh rate will be described here.

The eye strain is divided into two categories: nerve strain and musclestrain. The nerve strain is caused by prolonged looking at light emittedfrom a liquid crystal display device or blinking images. This is becausethe brightness stimulates and fatigues the retina and nerve of the eyeand the brain. The muscle strain is caused by overuse of a ciliarymuscle which works for adjusting the focus.

FIG. 30A is a schematic diagram illustrating display of a conventionalliquid crystal display device. As shown in FIG. 30A, for the display ofthe conventional liquid crystal display device, image rewriting isperformed 60 times per second. A prolonged looking at such a screenmight stimulate a retina, optic nerves, and a brain of a user and leadto eye strain.

In one embodiment of the present invention, a transistor with anextremely low off-state current (e.g., a transistor using an oxidesemiconductor) is used in a pixel portion of a liquid crystal displaydevice. In addition, the liquid crystal element has a large-areacapacitor. With these components, leakage of electrical chargesaccumulated on the capacitor can be suppressed, whereby the luminance ofa liquid crystal display device can be kept even at a lower framefrequency.

That is, as shown in FIG. 30B, an image can be rewritten as lessfrequently as once every five seconds, for example. This enables theuser to see the same one image as long as possible, so that flickers onthe screen recognized by the user are reduced. Consequently, a stimulusto the retina or the nerve of an eye or the brain of the user isrelieved, resulting in less nervous fatigue.

One embodiment of the present invention can provide an eye-friendlyliquid crystal display device.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments.

Embodiment 8

In this embodiment, structural examples of electronic devices each usinga display device of one embodiment of the present invention will bedescribed. In addition, in this embodiment, a display module using adisplay device of one embodiment of the present invention will bedescribed with reference to FIG. 31.

In a display module 8000 in FIG. 31, a touch panel 8004 connected to anFPC 8003, a display panel 8006 connected to an FPC 8005, a backlightunit 8007, a frame 8009, a printed board 8010, and a battery 8011 areprovided between an upper cover 8001 and a lower cover 8002. Note thatthe backlight unit 8007, the battery 8011, the touch panel 8004, and thelike are not provided in some cases.

The display device of one embodiment of the present invention can beused for the display panel 8006, for example.

The shapes and sizes of the upper cover 8001 and the lower cover 8002can be changed as appropriate in accordance with the sizes of the touchpanel 8004 and the display panel 8006.

The touch panel 8004 can be a resistive touch panel or a capacitivetouch panel and may be formed so as to overlap with the display panel8006. A counter substrate (sealing substrate) of the display panel 8006can have a touch panel function. A photosensor may be provided in eachpixel of the display panel 8006 to form an optical touch panel. Anelectrode for a touch sensor may be provided in each pixel of thedisplay panel 8006 so that a capacitive touch panel is obtained.

The backlight unit 8007 includes a light source 8008. The light source8008 may be provided at an end portion of the backlight unit 8007 and alight diffusing plate may be used.

The frame 8009 protects the display panel 8006 and also functions as anelectromagnetic shield for blocking electromagnetic waves generated bythe operation of the printed board 8010. The frame 8009 can function asa radiator plate too.

The printed board 8010 is provided with a power supply circuit and asignal processing circuit for outputting a video signal and a clocksignal. As a power source for supplying power to the power supplycircuit, an external commercial power source or a power source using thebattery 8011 provided separately may be used. The battery 8011 can beomitted in the case of using a commercial power source.

The display module 8000 may be additionally provided with a member suchas a polarizing plate, a retardation plate, or a prism sheet.

FIGS. 32A to 32D are each an external view of an electronic deviceincluding a display device of one embodiment of the present invention.

Examples of electronic devices are a television set (also referred to asa television or a television receiver), a monitor of a computer or thelike, a camera such as a digital camera or a digital video camera, adigital photo frame, a mobile phone handset (also referred to as amobile phone or a mobile phone device), a portable game machine, aportable information terminal, an audio reproducing device, alarge-sized game machine such as a pachinko machine, and the like.

FIG. 32A illustrates a portable information terminal including a mainbody 1001, a housing 1002, display portions 1003 a and 1003 b, and thelike. The display portion 1003 b is a touch panel. By touching akeyboard button 1004 displayed on the display portion 1003 b, a screencan be operated, and text can be input. It is needless to say that thedisplay portion 1003 a may be a touch panel. A liquid crystal panel oran organic light-emitting panel is fabricated using any of thetransistors described in the above embodiments as a switching elementand used in the display portion 1003 a or 1003 b, whereby a highlyreliable portable information terminal can be provided.

The portable information terminal illustrated in FIG. 32A can have afunction of displaying a variety of information (e.g., a still image, amoving image, and a text image); a function of displaying a calendar, adate, the time, and the like on the display portion; a function ofoperating or editing the information displayed on the display portion; afunction of controlling processing by various kinds of software(programs); and the like. Furthermore, an external connection terminal(an earphone terminal, a USB terminal, or the like), a recording mediuminsertion portion, and the like may be provided on the back surface orthe side surface of the housing.

The portable information terminal illustrated in FIG. 32A may transmitand receive data wirelessly. Through wireless communication, desiredbook data or the like can be purchased and downloaded from an e-bookserver.

FIG. 32B illustrates a portable music player including, in a main body1021, a display portion 1023, a fixing portion 1022 with which theportable music player can be worn on the ear, a speaker, an operationbutton 1024, an external memory slot 1025, and the like. A liquidcrystal panel or an organic light-emitting panel is fabricated using anyof the transistors described in the above embodiments as a switchingelement and used in the display portion 1023, whereby a highly reliableportable music player can be provided.

Furthermore, when the portable music player illustrated in FIG. 32B hasan antenna, a microphone function, or a wireless communication functionand is used with a mobile phone, a user can talk on the phone wirelesslyin a hands-free way while driving a car or the like.

FIG. 32C illustrates a mobile phone, which includes two housings, ahousing 1030 and a housing 1031. The housing 1031 includes a displaypanel 1032, a speaker 1033, a microphone 1034, a pointing device 1036, acamera 1037, an external connection terminal 1038, and the like. Thehousing 1030 is provided with a solar cell 1040 for charging the mobilephone, an external memory slot 1041, and the like. In addition, anantenna is incorporated in the housing 1031. Any of the transistorsdescribed in the above embodiments is used in the display panel 1032,whereby a highly reliable mobile phone can be provided.

Furthermore, the display panel 1032 includes a touch panel. A pluralityof operation keys 1035 that are displayed as images are indicated bydotted lines in FIG. 32C. Note that a boosting circuit by which voltageoutput from the solar cell 1040 is increased to be sufficiently high foreach circuit is also included.

In the display panel 1032, the direction of display is changed asappropriate depending on the application mode. In addition, the mobilephone has the camera 1037 and the display panel 1032 on the same surfaceside, and thus it can be used as a video phone. The speaker 1033 and themicrophone 1034 can be used for videophone calls, recording, and playingsound, etc., as well as voice calls. Moreover, the housings 1030 and1031 in a state where they are developed as illustrated in FIG. 32C canshift, to a state where one is lapped over the other by sliding.Therefore, the size of the mobile phone can be reduced, which makes themobile phone suitable for being carried around.

The external connection terminal 1038 can be connected to an AC adaptorand a variety of cables such as a USB cable, whereby charging and datacommunication with a personal computer or the like are possible.Furthermore by inserting a recording medium into the external memoryslot 1041, a larger amount of data can be stored and moved.

In addition, in addition to the above functions, an infraredcommunication function, a television reception function, or the like maybe provided.

FIG. 32D illustrates an example of a television set. In a television set1050, a display portion 1053 is incorporated in a housing 1051. Imagescan be displayed on the display portion 1053. Moreover, a CPU isincorporated in a stand 1055 supporting the housing 1051. Any of thetransistors described in the above embodiments is used in the displayportion 1053 and the CPU, whereby the television set 1050 can have highreliability.

The television set 1050 can be operated with an operation switch of thehousing 1051 or a separate remote controller. In addition, the remotecontroller may be provided with a display portion for displaying dataoutput from the remote controller.

Note that the television set 1050 is provided with a receiver, a modem,and the like. With the use of the receiver, general televisionbroadcasting can be received. Moreover, when the television set isconnected to a communication network with or without wires via themodem, one-way (from a sender to a receiver) or two-way (between asender and a receiver or between receivers) information communicationcan be performed.

Furthermore, the television set 1050 is provided with an externalconnection terminal 1054, a storage medium recording and reproducingportion 1052, and an external memory slot. The external connectionterminal 1054 can be connected to various types of cables such as a USBcable, and data communication with a personal computer or the like ispossible. A disk storage medium is inserted into the storage mediumrecording and reproducing portion 1052, and reading data stored in thestorage medium and writing data to the storage medium can be performed.In addition, an image, a video, or the like stored as data in anexternal memory 1056 inserted into the external memory slot can bedisplayed on the display portion 1053.

Furthermore, in the case where the off-state leakage current of thetransistor described in the above embodiments is extremely small, whenthe transistor is used in the external memory 1056 or the CPU, thetelevision set 1050 can have high reliability and sufficiently reducedpower consumption.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments.

REFERENCE NUMERALS

-   10: transistor, 10 a: transistor, 10 b: transistor, 11: substrate,    12: conductive film, 13: conductive film, 14: gate insulating film,    15: nitride insulating film, 16: oxide insulating film, 17: oxide    insulating film, 18: oxide semiconductor film, 19 a: oxide    semiconductor film, 19 b: pixel electrode, 19 c: oxide semiconductor    film, 19 d: film, 19 f: oxide semiconductor film, 19 g: oxide    semiconductor film, 20: conductive film, 21 a: conductive film, 21    b: conductive film, 21 b_1: region, 21 b_2: region, 21 c: common    line, 21 d: conductive film, 21 e: conductive film, 21 f: conductive    film, 21 g: conductive film, 21 h: conductive film, 21 i: conductive    film, 22: oxide insulating film, 23: oxide insulating film, 24:    oxide insulating film, 25: oxide insulating film, 26: nitride    insulating film, 27: nitride insulating film, 28: conductive film,    29: common electrode, 29 a: common electrode, 29 a_1: region, 29    a_2: region, 29 b: conductive film, 29 c: conductive film, 29 d:    conductive film, 30: inorganic insulating film, 30 a: inorganic    insulating film, 31: organic insulating film, 31 a: organic    insulating film, 31 b: organic insulating film, 31 c: organic    insulating film, 33: alignment film, 37 a: multilayer film, 37 b:    multilayer film, 38 a: multilayer film, 38 b: multilayer film, 39 a:    oxide semiconductor film, 39 b: oxide semiconductor film, 40:    opening, 41: opening, 41 a: opening, 42: opening, 49 a: oxide    semiconductor film, 49 b: oxide semiconductor film, 70: electron gun    chamber, 72: optical system, 74: sample chamber, 76: optical system,    78: camera, 80: observation chamber, 82: film chamber, 84: electron,    88: substance, 92: fluorescent plate, 101: pixel portion, 102:    transistor, 102 a: transistor, 102 b: transistor, 102 c: transistor,    103: pixel, 103 a: pixel, 103 b: pixel, 103 c: pixel, 104: scan line    driver circuit, 105: capacitor, 105 a: capacitor, 105 b: capacitor,    105 c: capacitor, 106: signal line driver circuit, 107: scan line,    109: signal line, 115: capacitor line, 121: liquid crystal element,    131: light-emitting element, 133: transistor, 135: transistor, 137:    wiring, 139: wiring, 141: wiring, 320: liquid crystal layer, 322:    liquid crystal element, 322 a: liquid crystal element, 342:    substrate, 344: light-blocking film, 346: coloring film, 348:    insulating film, 350: conductive film, 352: alignment film, 1001:    main body, 1002: housing, 1003 a: display portion, 1003 b: display    portion, 1004: keyboard button, 1021: main body, 1022: fixing    portion, 1023: display portion, 1024: operation button, 1025:    external memory slot, 1030: housing, 1031: housing, 1032: display    panel, 1033: speaker, 1034: microphone, 1035: operation key, 1036:    pointing device, 1037: camera, 1038: external connection terminal,    1040: solar cell, 1041: external memory slot, 1050: television set,    1051: housing, 1052: storage medium recording and reproducing    portion, 1053: display portion, 1054: external connection terminal,    1055: stand, 1056: external memory, 8000: display module, 8001:    upper cover, 8002: lower cover, 8003: FPC, 8004: touch panel, 8005:    FPC, 8006: display panel, 8007: backlight unit, 8008: light source,    8009: frame, 8010, printed board, and 8011: battery.

This application is based on Japanese Patent Application serial no.2013-219516 filed with Japan Patent Office on Oct. 22, 2013 and JapanesePatent Application serial no. 2014-047260 filed with Japan Patent Officeon Mar. 11, 2014, the entire contents of which are hereby incorporatedby reference.

1. A display device comprising: a transistor over a first substrate; aninorganic insulating film over the transistor; an organic insulatingfilm on and in contact with the inorganic insulating film; and a pixelelectrode on and in contact with the inorganic insulating film, thepixel electrode electrically connected to the transistor, wherein thetransistor comprises: a gate electrode over the first substrate; anoxide semiconductor film over the gate electrode; and a gate insulatingfilm between the gate electrode and the oxide semiconductor film,wherein an upper surface of the oxide semiconductor film is in contactwith the inorganic insulating film, wherein the organic insulating filmoverlaps with the oxide semiconductor film with the inorganic insulatingfilm provided therebetween, and wherein an end portion of the gateelectrode is positioned on an outer side of an end portion of theorganic insulating film.
 2. The display device according to claim 1,further comprising: a second substrate overlapping with the firstsubstrate; and a liquid crystal layer between the organic insulatingfilm and the second substrate, wherein the transistor and the organicinsulating film are provided between the first substrate and the secondsubstrate.
 3. The display device according to claim 1, furthercomprising: a second substrate overlapping with the first substrate; anda liquid crystal layer between the pixel electrode and the secondsubstrate, wherein the transistor and the organic insulating film areprovided between the first substrate and the second substrate, andwherein the liquid crystal layer is not provided between the organicinsulating film and the second substrate.
 4. The display deviceaccording to claim 1, wherein a thickness of the organic insulating filmis greater than or equal to 500 nm and less than or equal to 10 μm. 5.The display device according to claim 1, wherein the inorganicinsulating film comprises an oxide insulating film in contact with theupper surface of the oxide semiconductor film and a nitride insulatingfilm on and in contact with the oxide insulating film.
 6. The displaydevice according to claim 1, wherein, in a plan view, the oxidesemiconductor film is the organic insulating film completely overlapswith the oxide semiconductor film.
 7. A display device comprising: atransistor over a first substrate; an inorganic insulating film over thetransistor; an organic insulating film on and in contact with theinorganic insulating film; a pixel electrode electrically connected tothe transistor; and a capacitor electrically connected to thetransistor, wherein the transistor comprises: a gate electrode over thefirst substrate; an oxide semiconductor film over the gate electrode;and a gate insulating film between the gate electrode and the oxidesemiconductor film, wherein an upper surface of the oxide semiconductorfilm is in contact with the inorganic insulating film, wherein theorganic insulating film overlaps with the oxide semiconductor film withthe inorganic insulating film provided therebetween, wherein an endportion of the gate electrode is positioned on an outer side of an endportion of the organic insulating film, wherein the capacitor comprisesthe pixel electrode, the inorganic insulating film and a metal oxidefilm, wherein the pixel electrode comprises a light-transmittingconductive material and the pixel electrode overlaps with the metaloxide film with the inorganic insulating film provided therebetween, andwherein the metal oxide film contains the same metal element as theoxide semiconductor film and an upper surface of the metal oxide film isin contact with the inorganic insulating film.
 8. The display deviceaccording to claim 7, further comprising: a second substrate overlappingwith the first substrate; and a liquid crystal layer between the organicinsulating film and the second substrate, wherein the transistor and theorganic insulating film are provided between the first substrate and thesecond substrate.
 9. The display device according to claim 7, furthercomprising: a second substrate overlapping with the first substrate; anda liquid crystal layer between the pixel electrode and the secondsubstrate, wherein the transistor and the organic insulating film areprovided between the first substrate and the second substrate, andwherein the liquid crystal layer is not provided between the organicinsulating film and the second substrate.
 10. The display deviceaccording to claim 7, wherein a thickness of the organic insulating filmis greater than or equal to 500 nm and less than or equal to 10 μm. 11.The display device according to claim 7, wherein the inorganicinsulating film comprises an oxide insulating film in contact with theupper surface of the oxide semiconductor film and a nitride insulatingfilm on and in contact with the oxide insulating film.
 12. The displaydevice according to claim 7, wherein, in a plan view, the organicinsulating film completely overlaps with the oxide semiconductor film.13. The display device according to claim 7, further comprising, a pairof electrodes in contact with the oxide semiconductor film, wherein oneof the pair of electrodes electrically connected to the pixel electrode.14. A display device comprising: a transistor over a first substrate; aninorganic insulating film over the transistor; an organic insulatingfilm on and in contact with the inorganic insulating film; a pixelelectrode electrically connected to the transistor; a capacitorelectrically connected to the transistor, wherein the transistorcomprises: a gate electrode over the first substrate; an oxidesemiconductor film over the gate electrode; and a gate insulating filmbetween the gate electrode and the oxide semiconductor film, wherein anupper surface of the oxide semiconductor film is in contact with theinorganic insulating film, wherein the organic insulating film overlapswith the oxide semiconductor film with the inorganic insulating filmprovided therebetween, wherein an end portion of the gate electrode ispositioned on an outer side of an end portion of the organic insulatingfilm, wherein the capacitor comprises the pixel electrode, the inorganicinsulating film and a light-transmitting conductive film, wherein thepixel electrode is provided over the gate insulating film and containsthe same metal element as the oxide semiconductor film, and wherein thelight-transmitting conductive film overlaps with the pixel electrodewith the inorganic insulating film provided therebetween and thelight-transmitting conductive film functions as a common electrode. 15.The display device according to claim 14, further comprising: a secondsubstrate overlapping with the first substrate; and a liquid crystallayer between the organic insulating film and the second substrate,wherein the transistor and the organic insulating film are providedbetween the first substrate and the second substrate.
 16. The displaydevice according to claim 14, further comprising: a second substrateoverlapping with the first substrate; and a liquid crystal layer betweenthe pixel electrode and the second substrate, wherein the transistor andthe organic insulating film are provided between the first substrate andthe second substrate, and wherein the liquid crystal layer is notprovided between the organic insulating film and the second substrate.17. The display device according to claim 14 wherein a thickness of theorganic insulating film is greater than or equal to 500 nm and less thanor equal to 10 μm.
 18. The display device according to claim 14, whereinthe inorganic insulating film comprises an oxide insulating film incontact with the upper surface of the oxide semiconductor film and anitride insulating film on and in contact with the oxide insulatingfilm.
 19. The display device according to claim 14, wherein, in a planview, the organic insulating film completely overlaps with the oxidesemiconductor film.
 20. The display device according to claim 14,further comprising, a pair of electrodes in contact with the oxidesemiconductor film, wherein one of the pair of electrodes electricallyconnected to the pixel electrode.